[单选题]
At what rate are photons eme t5ffl:uwo- 0itted by a 50-W sodium vapor lamp?2k wci zit3z2 ml(539jypi 2dk (Assume that the lamp's light is monochromatic and of wavelengk izmtwcpd3jl(k35y2i92i 2z th $589 \; nm$.)
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