[单选题]
Two light sources are said zfma(z )e v+ eh0oac-.hm-mz4to be coherentvudxs5( - is+b 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.