Which condition is critical for the Michelson-Morley experiment?

[Shark 774]

Which one of the statements below specifies the critical basis of the apparatus used in the Michelson/Morley experiment?
A) The half-silvered mirror must reflect exactly half the light.
B) Mirror 1 and mirror 2 must be identical.
C) The distance traveled by light using either path must be equal.
D) The components must not move relative to each other.

I don't think A or B are right. If the different distances were known C would be fine. If the relative speeds and distances between parts were known then D should also be fine, I would think. Any ideas?

 

[ideasrule]

D is not fine. If the components of the interferometer moved, and you observed a shift in the interference pattern, how would you know whether the shift was due to the components moving or due to the aether changing direction relative to the device? In theory, you can precisely measure the movements of the interferometer's components and account for them. In practice, it's impossible to measure sub-micrometer movements that precisely.