[单选题]
Two light sources are sg,7;s e6( zu8b0djol aw8dz uk:bi kb,aid to be coherej/tzzd 5v 4hak:od52 ant 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.