Based on the question provided, there are no specifics or additional information given regarding the distance covered by the plane or the time it took for the descent. We need to know, at minimum, either the time and distance of the descent or any related variables to help calculate the speed of the plane.
Speed is calculated using the formula: speed = distance/time. Without the pertinent distance or time data for the plane's descent, there is not enough information to solve the problem. Therefore, none of the provided answer choices A, B, C, or D can be justified with the given details.
Hence, the answer to the question "How fast did the plane descend?" with the given conditions is - "The problem cannot be solved due to insufficient information."