8-9 can someone help me with this, I answered 8 but for 9 I don't know, if you can also verify my answer for 8

A diagram of a Michelson-type apparatus is displayed below. The stationary mirror is 50.0 km away from the observing station. 50.0 km 8. The lowest (non-zero) frequency of rotation for which an image of the light source will be seen is a.bc x 10 Hz. The values of a, b, c and d are _ and (Record your answer in the numerical-response section below.) Your answer: 9. If it is not possible to build a motor capable of spining the apparatus fast enough, which of the following adjustments would allow for a slower rotating apparatus to be used? O move the stationary mirror closer to the apparatus use a longer wavelength of light use a rotating apparatus with 6 sides O use a glass prism (n=1.50) between the laser and the rotating mirror