[单选题]
At what rate are photon3dfpe+xsnq0k*h9xy2 fe2z 3) dhlmx +s emitted b(aa.bxm3b+ jkc.5m h oy a 50-W sodium vapor lamp? (Assume that the lamp's light is monochromatic and of wac h.jaxb+.b3(mo 5km avelength $589 \; nm$.)
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
