[单选题]
At what rate are photons * ff. w f(a-9xf/kjpabemitted by a 50-W sod:vm0y6 g p-vbnium vapor lamp? (Assume that the lampg6b: vn ym0-vp's light is monochromatic 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
