[单选题]
According to the sele p 4 ;gwg/5n*tk4knrijction rule, when a photon is elp kg *ge5 hpexea 5:0-beh4:imitted or absorbed, transitions can only occu4*gaee-lh0g pp:hx5 keieb 5:r between state with values of l that differ by
A. four units.
B. three units.
C. two units.
D. one unit.
