[单选题]
According to the selection rule, when a pho6gu:..xxbut h ton2 1 mkd-kx8 poiwq,zj4 is emitted or absorbed, transitions can only occur between state with values of l that diffej2d4p wx,1 k8ik o-qzmr by
A. four units.
B. three units.
C. two units.
D. one unit.
