[单选题]
In a ruby laser, an electron jumps from a higher energy levelq56xwfpqef ap3+ (+ty tov2w;htoq/m i5 a lower one. If the energy difference between th;25 i otvqh/mwe two levels is 1.8 eV, what is the wavelength of the emitted photon?
A. $469 \; nm$
B. $649 \; nm$
C. $694 \; nm$
D. $964 \; nm$
