While emitting light of proper frequency f0, a source moves to the right with speed c/4 relative to reference frame S. A detector, to the left of the source, measures the frequency to be f, which is greater than f0. This means:
A. the detector is moving to the right with a speed that is greater than c/4 relative to S
B. the detector is moving to the right with a speed that is less than c/4 relative to S
C. the detector is moving to the left with a speed that is greater than c/4 relative to S
D. the detector is moving to the left with a speed that is less than c/4 relative to S
E. the detector is not moving