[单选题]
Two light sources are said p 0k kl5:ww4sph 8:epx ih c4u,w 2p p2kgxoga5pi(/;wto be coherent 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.