[单选题]
According to the selection rule, when a photon is e77l0+rq km:2pm6s) ga*crq hfpfw 5clmitted or absorbedv,m;e /7wjvd b, transitions can only occur bemdb,7wevv; j/ tween state with values of l that differ by
A. four units.
B. three units.
C. two units.
D. one unit.
