[单选题]
Two light sources are said to be coher:nw0gtecypr xg4; 9w-3 zcusuy 0b7d.entofbt 0/y61cypi 0vu*y if they
A. are of the same frequency.
B. are of the same frequency, and maintain a constant phase difference.
C. are of the same amplitude, and maintain a constant phase difference.
D. are of the same frequency and amplitude.