[单选题]
Two light sources are said to be cvr:n0db)s7yh g9 rf w)ohere0*izu(/ess9dr x ywr:;t2u kzo)d 4kent 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.