[单选题]
In a ruby laser, an electron f i;3 vonu8;vv 1psrb+ fds+/gjumps from a higherh q24c:olhgin v8zvo0k 46yz; energy level to a lower one. If the energy difference vvh6ico ; yl0 gkq4:48zzno 2hbetween 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. $964 \; nm$
