[单选题]
At what rate are photons emitted by a 50-W sna ;x)7dh le 3tkd9w+:,,9zqu wv gknsodi aiz;0zxjjq 48;n*n+eyyx5vjum vapor lamp? (Assume that the lamp's light is monochromatic and of wavex ;eaizqvj40yj+ ;5 y*n zx8njlength $589 \; nm$.)
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
