[单选题]
At what rate are photons emil,h)xni eai4 htanqy 7 k+uua4s)(a/1ttu*v9)w; myy zped by a 50-W sodium vapor lamp? (Assume that the lamp's light is monoc )wz ;mv9y*ypuhromatic 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