[单选题]
According to the selection rule, when a phot aqx16h q*f)nhon is emitted or abszgo8zy( /5 c b45uekylorbed, transitions can only occur between state with values of c/l 4oyu5zk8zebgy 5( l that differ by
A. four units.
B. three units.
C. two units.
D. one unit.
