solve please

Albert Michelson used a rotating mirror and a plane mirror, as shown in the diagram, to determine precisely the speed of light. This allowed a pulse of light to be reflected and observed at the detector only when the rotating mirror rotated at a proper frequency. Determine the separation distance between Michelson's rotating 8-sided mirror and the plane mirror if the mirror must be rotating at 624.5 Hz to allow the observer to detect the reflected light pulse. A. 15.0 km B. 30.0 km C. 60.0 km D. 120 km