[单选题]
In a ruby laser, an electron jw 1+hptfmre)df :7lu2 um,qj*gym2. bemps from a higher energy level to a lower one. If the energy difference bey* ,gqjmm. e2btween the two levels is 1.8 eV, what is the wavelength of the emitted photon?
A. $469 \; nm$
B. $649 \; nm$
C. $694 \; nm$
D. $964 \; nm$
