[单选题]
At what rate are photons emitted by - jv9d *p8,vagsjmh r+kflni24 r 1 hb hq;2k,gho m-u7j7fh6oa 50-W sodium vapor lamp? (Assume that the lamp's light is monochromatic and of wavele4n hok jrlh2f -m67;2b kqo7,if hu1hgngth $589 \; nm$.)
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
