[单选题]
At what rate are pho9pkkqe(e k*.yqkb :0itons emitted by a 50-W sodium vapor lamp? (Assu;ua-mnnoyq gnmhj/ xsf*155) me that the lamp's light is monochromaticmn-;ygnshmxf 5) au* 15 /onjq 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