In the Binary Pulsar Hercules X-1 the Periodic Doppler Shift

Multiple Choice

In the binary pulsar Hercules X-1 the periodic Doppler shift, once every 1.7 days, was discovered rather quickly. Even though the binary pulsar Centaurus X-3 was discovered earlier, its Doppler shift was discovered only much later. What might be a reason for this?

A) The orbital plane of Centaurus X-3 is in the plane of the sky and therefore has no radial motion and no Doppler shift.
B) Centaurus X-3 is too far away to see the Doppler shift easily.
C) The orbital period of Centaurus X-3, 2087 days, suggests that it moves relatively slowly.
D) The binary companion of Centaurus X-3 is a black hole that distorts the pattern of its radiation output.