[单选题]
According to the selection rule, when a photon iott4epl0)cw t;h g-; sdad0o,s emitted or absorbed, tu+hnyk z,dg/ .s7t b+oransitions can only occur between state with values of l that differ b,+sknd+ oz/u7h .ty gby
A. four units.
B. three units.
C. two units.
D. one unit.
