While Emitting Light of Proper Frequency F₀, a Source Moves

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