21. Testimony by the investigator is

C is correct. If scientific, technical, or other specialized knowledge will assist the trier of fact to understand the evidence or to determine a fact in issue, a witness qualified as an expert by knowledge, skill, experience, training, or education may testify thereto in the form of an opinion. The police accident investigator's opinion was based upon sufficient facts, was the product of reliable principles and methods, and the investigator has applied the principles and methods reliably to the facts of the case. The testimony of the investigator, because he has sufficient expertise to express an opinion, is admissible.

A is incorrect. It is a misstatement of the law. There certainly can be both lay and expert opinion on the same issue.

B is incorrect. The investigator relied on his training and experience, as well as his examination of the physical evidence, to reach his opinion on the speed of the train, which is the proper standard for the admissibility of his opinion.

D is incorrect. The ability of the investigator to give an opinion on the speed of the train is not conditioned on the plaintiff's first introducing opinion evidence on speed. Even if the plaintiff had not entered any opinion evidence on speed, the investigator's testimony would still be admissible.