[单选题]
At what rate are photons:ohrj.h -xk9e emitted by a 50-W sodi4 sdzg.hes j(-um vapor lamp? (Assume that the lam( 4hesd-sg zj.p's light is monochromatic and of wavelength $589 \; nm$.)
A. $1.0 \times 10^{20}/$s
B. $1.5 \times 10^{20}/$s
C. $2.0 \times 10^{20}/$s
D. $2.5 \times 10^{20}/$s
