Question: The hydrogen lines observed in the spectrum of the quasar 3C9 are shifted so far into the red that their wavelengths are three times as long as those observed in the light from hydrogen atoms at rest in a laboratory on Earth. If it is assumed that the shift is entirely due to the relative motion of 3C9 and Earth, the relative speed of the quasar is
A: 2c
B: c
C: 0.8c
D: 0.5c
Please eliminate two incorrect options first, then think it step by step and choose the most proper one option.