[单选题]
According to the selection rule (1l:nc kvhy(evm9o):4jbif *im)p d m, when a photon is emittjooqj/c ry3 :3yc9ne 2oil2(ted or absorbed, transitionsiy :ooce33n(2rqjc9yo2l/j t can only occur between state with values of l that differ by
A. four units.
B. three units.
C. two units.
D. one unit.
