The roof on a city's new assembly hall collapsed after a snowstorm. The city filed in federal court a civil action against the architect and builder, claiming negligence in design, construction, and supervision. The city's attorney retained an expert construction engineer to assist with the trial and provide opinions.

Which of the following statements is correct regarding the defendants' right to depose the engineer?

A. If the city expects to call the engineer as an expert witness at trial, the defendants may depose the engineer if they properly notice a deposition and have a subpoena served on the engineer.
B. If the city expects to call the engineer as an expert witness at trial, the defendants may depose the engineer only if they obtain a court order.
C. If the city does not expect to call the engineer as an expert witness at trial, then under no circumstances may the defendants depose him.
D. Regardless of whether the city expects to call the engineer as an expert witness at trial, the defendants may not depose him because his opinions are protected as work product prepared in anticipation of litigation and thus not subject to discovery absent a showing of substantial need for the information and inability to obtain its substantial equivalent by other means.