When shooting by aerial photography, if the focal length of the lens of the camera used for shooting is 50mm, the distance (image distance) from the film to the lens should be () a.

The lens of a camera is equivalent to a convex lens. The focal length of the lens is 50mm, and the image distance between the film and the lens is 2f > v > f, so the distance between the film and the lens is more than 50mm and less than100 mm. Because the aerial photographer shoots from high altitude, the image is almost in focus, just slightly larger than the focal length, that is, slightly larger than 50 mm

So choose B.