[单选题]
According to the selectnq)t*etj;1 evlsrt 0v* tkz0 4iy9qe (x. qrx9hon rule, when a photon is emitted or absorbed, transitions can only occur between state with values of l that diffexehyxr (q.99q r by
A. four units.
B. three units.
C. two units.
D. one unit.
