[单选题]
At what rate are photons emitted by swqw-(fno8td)2fl w0 fm75v6kl yr0oa (zc:e,ey9 -vew b7cc k50-W sodium vapor lamp? (Assume that the lamp's light is monochroma ck wey7vbzce9ec -:(,tic 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