[单选题]
At what rate are photons emivpxv d926 x9zu o2iu7rtted by a 50-W sodium vapot.wu*p/( zwm / owcy3vr lamp? (Assume that the lamp's light is monochromatic a/u .(pmtyvz*3/oc wwwnd 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