[单选题]
At what rate are photons emitted by a 5nq. n.iy3fulg oso /ku.m/7.v0-W sod l /e5:v.4-o4jc a/qcpdkcds57 sdc hmium vapor lamp? (Assume that the lamp's light is mon/sc4pc.:dol acqj/55kvd c4 7d me-hsochromatic 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
