Transportation Company’s liability to plaintiffs has been finally determined. It did not appeal from the judgments which plaintiffs recovered against it on account of the injuries they sustained in the bus accident in suit. This appeal presents the questions (1) whether Transportation Company offered any evidence tending to show that negligence on the part of Coach Company caused the bus accident in which plaintiffs were injured; and (2), if so, whether Coach Company’s liability to Transportation Company is for indemnity or contribution.
 Transportation Company, a common carrier, is not an insurer of its passengers; it is liable only for negligence proximately causing injury to them. However, a carrier owes to the passengers whom it undertakes to transport “the highest degree of care for their safety so far as is consistent with the practical operations and conduct of its business.” White v. Chappell, 219 N.C. 652, 659, 14 S.E. 2d 843, 847 (1941). See Harris v. Greyhound Corp., 243 N.C. 346, 90 S.E. 2d 710 (1956); Garvey v. Greyhound Corp., 228 N.C. 166, 45 S.E. 2d 58 (1947); 14 Am. Jur. 2d Carriers § 918 (1964).
*744  The high degree of care, which a carrier operating under a public franchise owes to its passengers, is a nondelegable duty. See Pennsylvania Co. v. Roy, 102 U.S. 451, 26 L.Ed. 141 (1880); Dixie Stage Lines v. Anderson, 222 Ala. 673, 134 So. 23 (1931); Colton v. Ship-by-Truck Co., 337 Mo. 280, 85 S.W. 2d 80 (1935); Simpson v. Gray Line Co., 226 Ore. 71, 358 P. 2d 516 (1961). See also Morgan v. Chesapeake & Ohio Ry. Co., 127 Ky. 433, 105 S.W. 961 (1907); Western Maryland R. R. v. State, 95 Md. 637, 53 A. 969 (1902); Virgil v. Riss & Co., 241 S.W. 2d 96 (Mo. App. 1951); Prosser, Law of Torts 470 (4th ed. 1971); 41 Am. Jur. 2d Independent Contractors § 39 (1968). “[A] passenger who sustains an injury by reason of the fact that the obligatory measure of care was not exercised is entitled to hold the carrier responsible, although the conditions or occurrences which caused the injury resulted from the negligence of an independent contractor.” Annot., 29 A.L.R. 736, 784 (1924).
[3, 4] “Among the duties falling. upon a common carrier of passengers are the important ones of providing adequate conveyances with sufficiently strong and serviceable equipment for the safe transportation of its passengers, and of inspecting such conveyances and equipment at proper intervals and keeping them in good repair.” 14 Am. Jur. 2d Carriers § 1028 (1964). See 13 C.J.S. Carriers § 735 (1939). The purchase of equipment from a reputable source “does not relieve the carrier of the further duty to inspect and test the equipment or appliances, and hence where an accident results from a defect which might have been discovered by a proper test made by the carrier, it is liable therefor.” 14 Am. Jur. 2d Carriers § 1030 (1964). Nor may a carrier relieve itself of the duty to exercise the highest degree of care to provide safe buses by leasing its transportation facilities from another carrier or corporation which has contracted to furnish and keep such equipment in proper condition. “[T]he carrier cannot delegate the performance of this duty and escape liability for its nonperformance.” 13 C.J.S. Carriers § 741(a) (1939). See 14 Am. Jur. 2d Carriers § 898 (1964).
 Thus, if the bus which Coach Company delivered to Gibbs on the morning of September 17, 1968 contained a pre-existing defect in the steering mechanism which could or should have been discovered by a proper inspection, and if the defect was the proximate cause or a proximate cause of plaintiffs’ injuries, Transportation Company would be liable to plaintiffs.
*745In Simpson v. Gray Line Co., supra, the plaintiff passenger was injured in a bus accident which occurred when a tire blew out. The defendant bus company attempted to avoid liability by showing that its tires were rented from a third party. The Oregon Supreme Court said: “[T]his fact [was] immaterial in determining the issue before the court. There is a duty upon the carrier to furnish tires that are fit for the intended use. . . .” Id. at 74, 358 P. 2d at 517. “[T]he defendant-carrier could not delegate its duty to a third party, i.e., to a tire company which supplied tires on a rental agreement. Pennsylvania Co. v. Roy, supra. . . .” Id. at 76, 358 P. 2d at 518.
[6, 7] Plaintiffs in this case made out a prima facie case of actionable negligence against Transportation Company by the introduction of evidence tending to show that they were injured when the bus in which they were passengers, without a prior collision or other apparent cause, ran off the highway into a ditch and struck a culvert. Greene v. Nichols, 274 N.C. 18, 161 S.E. 2d 521 (1968); Simpson v. Gray Line Co., supra; 2 Stansbury, North Carolina Evidence § 227 (Brandis rev. 1973); Annot., 79 A.L.R. 2d §§ 23(b), 31(a) (1961); 14 Am. Jur. 2d Carriers § 1161 (1964); 13 C.J.S. Carriers § 764(f)(4) (1939). Thus, plaintiffs would have been entitled to go to the jury as against Transportation Company without plaintiff Mann’s testimony that the bus driver consumed “soda and cake” while operating the bus and then threw or attempted to throw the bottle out the window just before the bus ran off the highway. The jury, therefore, was not required to accept this testimony in order to answer the issues in favor of plaintiffs. We, of course, cannot know upon what theory the jury answered the issues of negligence in favor of the plaintiffs. The judge’s charge is not in the record, but the presumption is that he submitted the case to the jury upon every theory which the evidence justified and “instructed correctly on every principle of law applicable to the facts.” Jones v. Mathis, 254 N.C. 421, 428, 119 S.E. 2d 200, 205 (1961).
The specific questions which determine this appeal are (1) whether Transportation Company offered evidence sufficient to sustain a finding that a defetít in the steering mechanism caused the leased bus to leave the highway; and (2), if so, whether Coach Company, in the exercise of proper care under the circumstances, could have discovered the'defect prior to the time it delivered the bus to Gibbs.
*746On Coach Company’s, motion for a directed verdict all the evidence which tends to support Transportation Company’s case against it must be taken as true and considered in the light most favorable to Transportation Company. Transportation Company is entitled to the benefit of every reasonable inference which may legitimately be drawn from the evidence. Kelly v. Harvester Co., 278 N.C. 153, 179 S.E. 2d 396 (1971); Bowen v. Gardner, 275 N.C. 363, 168 S.E. 2d 47 (1969); Phillips, 1970 Supplement to 2 McIntosh, N. C. Practice and Procedure § 1488.15. Therefore, on this motion, Gibbs’ statement that he neither threw nor attempted to throw a bottle from the bus and had no cake or soft drink on the bus at any time during the trip on which plaintiffs were injured must be accepted as true, and plaintiff Mann’s testimony that the bus ran off the road just as Gibbs prepared to throw a soda bottle out the window must be disregarded. It follows that, for the purpose of this motion, the only evidence of negligence on the part of Transportation Company is the fact that the bus suddenly left the highway in a curve.
Gibbs’ testimony negated any negligence in his manner of operating the bus. His statement that, as he approached the curve in which the bus left the road, he turned the steering wheel as usual but “the wheels did not answer to the steering wheel”; that although he kept turning the wheel to the left and applied his brakes, the bus kept to the right and into the ditch was corroborated by plaintiff Mann. She said that before the bus went into the ditch the driver was trying to pull it back onto the road but it would not come; that “he was turning the steering wheel to his left but the bus wouldn’t go back and the bus went on the shoulder. . . .”
The foregoing evidence tends to show that a defect in the steering mechanism caused the bus to go into the ditch and to negate Coach Company’s contentions (1) that the steering gear was damaged by its collision with the culvert, and (2) that “there was absolutely no evidence of any defect existing prior to the accident.”
The evidence does not show the exact date on which repairs began. Coach Company’s president, in answer to interrogatories not specifically directed to this point, said that the repairs were not begun until after 1 October 1968, and they were completed about 15 December 1968. It is a fair inference that repairs had not been begun at the time Transportation Company’s expert *747damage analyst, Mr. Jeffries, examined the bus on IT October 1968. At that time the whole right front section and the right side of the body was damaged and misaligned; the floor of the bus was buckled and the power steering cylinder disconnected from the extension of the steering arm. Nothing he saw suggested that any repairs had then been made or attempted.
Interrogatories answered by Coach Company’s president on 23 March 1972 disclosed that on the day after the accident the bus was towed from the scene of the accident to Coach Company’s garage in Norfolk. Coach Company found no missing parts from the steering mechanism at the scene.
On the day after the accident, in Norfolk, “a general inspection was made of all the damage to the bus, including the steering mechanism.” At that time Coach Company removed two bolts from the steering mechanism — two bolts from the booster flange. The record contains no explanation of why Coach Company removed these bolts. On 23 March 1972, at the time the interrogatories were answered, the bolts were in the possession- of Coach Company’s attorneys. They were in the possession of Coach Company’s supervisor of maintenance when Jeffries examined them on 23 September 1968.
Jeffries testified that the function of the two bolts was to unite the booster flange on the power steering cylinder and the flange on the steering arm. The small rings of steel entwined in the threads on these bolts and the wear in the holes in the flanges from which the bolts were removed caused Jeffries to conclude that the steel rings were threads stripped from the nuts which had been on the bolts; that in consequence of the stripping the nuts became loose, worked back and forth, and eventually sheared the cotter pin at the end of the bolt, thereby breaking the connection between the two flanges; and that this severance could or might have caused the steering system to fail when Gibbs attempted to steer the bus around the left curve.
It is apparent that, in phrasing the hypothetical question which elicited the foregoing opinion from Jeffries, counsel was observing the rule stated in 1 Stansbury, North Carolina Evidence § 137, at 453 (Brandis rev. 1973), that if the question relates to cause and effect an expert witness “should be asked whether in his opinion a particular event or condition could or might have produced the result in question, not whether it did produce such result.” This form of question clearly invited *748the argument, which Coach Company makes, that could or might have in Jeffries’ answers amounts to nothing more than his speculation as to possibilities. The situation here produced demonstrates the validity of Professor Henry Brandis’ comment that an expert witness should be allowed “to make a positive assertion of causation when that conforms to his true opinion, reserving ‘could’ and ‘might’ for occasions when he feels less certainty”; that if the expert witness, “though holding a more positive opinion, is forced to adopt the ‘could’ or ‘might’ formula, then the result is patently unjust, unless the more positive opinion may be said to be inherently incredible.” 1 Stansbury, North Carolina Evidence § 137, at 455 & n. 97 (Brandis rev. 1973). See also the comment of Justice Higgins in Apel v. Coach Co., 267 N.C. 25, 30, 147 S.E. 2d 566, 569-70 (1966). Cf. Service Co. v. Sales Co., 259 N.C. 400, 414, 131 S.E. 2d 9, 20 (1963).
 When a jury’s inquiry relates to cause and effect in a field where special knowledge is required to answer the question, the purpose of expert testimony is likely to be thwarted or perverted unless the expert witness is allowed to express a positive opinion (if he has one) on the subject. Here, however, Jeffries testified without objection that, in his opinion, the nuts on the bolts connecting the flanges in the steering mechanism were stripped and, in consequence, the bolts became loose and moved, back and forth, wearing the holes through which the bolts connected the flanges; that this movement in the flanges finally severed the cotter pin at the end of each bolt, breaking the connection between the power steering cylinder and the steering arm. Jeffries had previously testified that should these flanges become separated there would be no steering power available to the wheel.
[9, 10] We hold that sufficient evidence was produced at the trial to support a finding that a defective steering mechanism caused the bus to leave the highway. The remaining question is whether Transportation Company offered any evidence tending to show that Coach Company, by a proper inspection, could have discovered the defect prior to delivering the bus to Gibbs on 17 September 1968. The bus was a nine-year-old substitute bus. Its age and the purpose of its use intensified Coach Company’s duty to have it inspected carefully by a qualified mechanic before it was delivered to Transportation Company. This duty was imposed by law as well as,by its contract with Transportation Company, and its breach would render Coach'Company liable not only to Tranportation Company but also- to a third *749person injured thereby. Roberts v. Memorial Park, 281 N.C. 48, 187 S.E. 2d 721 (1972); Wilcox v. Motors Co., 269 N.C. 473, 153 S.E. 2d 76 (1967); Comment, Products Liability—Liability of the Bailor for Hire for Personal Injuries Caused, by Defective Goods, 51 N.C.L. Rev. 786 (1973); 1 Strong, N. C. Index 2d Automobiles § 23 (1967); 8 Am. Jur. 2d Automobiles and Highway Traffic §§ 663, 704 (1963); Annot., 46 A.L.R. 2d 407, 443 (1956).
 In response to a series of questions, had Jeffries been permitted to do so, he would have testified in substance as follows : Based upon his personal examination of the entire steering mechanism of the bus, including the bolts from the two separated flanges, in his opinion, the conditions which he found (and upon which he based his conclusion that a defect in the steering mechanism caused the bus to leave the highway) would have been visible to a competent mechanic prior to the time the bus was delivered to Gibbs; that the condition must have existed during, two or three thousand miles of operation before it could or might have caused the connection between the flanges to be broken and the steering to fail; that the condition would have been visible to a trained and competent mechanic “by the looseness between the two flanges of the steering cylinder and the steering rod, and the cracking of the dirt, dust and road accumulation in the joint which would signify that the joint was working, that is, moving, when it should be absolutely tight and stationary. The two pieces should not work or move against each other . . . and the nuts and bolt heads would have moved some small amount from their original position, leaving marks of the original position on the flanges.”
We hold that the foregoing evidence was competent and its rejection was error entitling Transportation Company to a new trial. Jeffries was an expert mechanic and analyst of damage to mechanical devices. As such his opinion on the matter under investigation could have been helpful to the jury. 1 Stansbury, North Carolina Evidence §§ 132-34 (Brandis rev. 1973). To show the nexus between a mechanical defect and an accident “[t]he most convincing evidence is an expert’s pinpointing the defect and giving his opinion on the precise cause of the accident after a thorough inspection.” Stewart v. Budget Rent-A-Car Corp., 52 Hawaii 71, 76, 470 P. 2d 240, 243 (1970).
 Upon the retrial of this case if the jury should find that the bus left the highway solely because of the negligent manner *750in which Gibbs- operated the bus, Coach Company would not be liable to Transportation Company in any amount. See Anderson v. Robinson, 275 N.C. 132, 165 S.E. 2d 502 (1969). If the jury should find that the bus left the highway because of a defect in the steering mechanism but that, in the exercise of proper care, Coach Company could not have discovered the defect prior to delivering the bus to Gibbs, Coach Company would have no liability to Transportation Company.
 If the jury should find that the bus left the highway because of a pre-existing defect in the steering mechanism which a competent mechanic could and should have discovered by a proper inspection of the bus prior to its delivery to Gibbs, Transportation Company would be entitled to recover indemnity from Coach Company. If this was the situation the two companies were not in equal fault. Coach Company,, an independent contractor, had contracted with Transportation Company to furnish it buses for the transportation of passengers and to maintain the buses.. However, Transportation Company, a common carrier, could not delegate to Coach Company its duty to use the highest degree of care to provide safe buses for its passengers. Under these circumstances, therefore, if Transportation Company was operating a bus with a pre-existing defect in the steering mechanism which Coach Company should have discovered, the negligence of Coach Company was primary and that of Transportation Company was secondary. See Hendricks v. Fay, Inc., 273 N.C. 59, 159 S.E. 2d 362 (1968); Edwards v. Hamill, 262 N.C. 528, 138 S.E. 2d 151 (1964); Steele v. Hauling Co., 260 N.C. 486, 133 S.E. 2d 197 (1963); Newsom v. Surratt, 237 N.C. 297, 74 S.E. 2d 732 (1953); Motor Lines v. Johnson, 231 N.C. 367, 57 S.E. 2d 888 (1950); Prosser, Law of Torts 310-13 (4th ed. 1971). Coach Company concedes that if there was a defect in the bus, Transportation Company had no knowledge of it.
The fact that defendant Coach Company, which plaintiffs alleged to be a joint tort-feasor with defendant Transportation Company, was dismissed from plaintiffs’ suits at the close of plaintiffs’ evidence was merely an adjudication that plaintiffs had not offered any evidence tending to establish actionable negligence on the part of Coach Company. The dismissal did not adjudicate that, as between Transportation Company and Coach Company, the two companies were not liable to plaintiffs as joint tort-feasors.
*751  If, from evidence adduced at the retrial, the jury should find Coach Company was guilty of actionable negligence in furnishing Transportation Company a defective bus; that Gibbs was negligent in the manner in which he operated the bus; and that the negligence of both concurred in proximately causing the accident in which plaintiffs were injured, Transportation Company would be entitled to contribution from Coach Company. See G.S. 1B-1, G.S. 1B-3 (1969); Pearsall v. Power Co., 258 N.C. 639, 129 S.E. 2d 217 (1963).
We are constrained to say that upon the next trial more carefully phrased hypothetical questions and more precise answers will greatly facilitate an understanding of the mechanical problems presented by the evidence.
The decision of the Court, of Appeals is reversed with directions that the cause be remanded to the Superior Court of Dare County for a retrial in accordance with the rules of law stated in this opinion.