[单选题]
At what rate are photons hd+ o/1d-jgg xemittzrgim8yqov.y4f (+4 jkwx2 *yed by a 50-W sodium vapor lamp? (Assume that the lamp's light is monochromatic and of wavelengthq.my vyj48y2k+(wgzf x*i or4 $589 \; nm$.)
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