[单选题]
At what rate are pho janr/./5yp ub k5-qh(c 2oxqaer,5g xtons emitted by a.q(-ykcbe s7p4/ ba 0qt.s ytl 50-W sodium vapor lamp? (Assume that the lamp's light is 0pay.7bq-(cl b4s stt/qy .kemonochromatic 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
