[单选题]
In a ruby laser, an electro yzi2lm gbe5 g1:ny)p)n jumps from a higher e h qd/5pqc)to:.uo eq/nergy level to a lower one. If the energy difference betwee /o.5dqqpe: qch/tuo) n 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. $964 \; nm$
