[单选题]
According to the selection rule, when a psu :: .dcg .m,mgvaur2 rlsb a(z 4o,kpu* 5gpsz);si:hoton is emitted or absorbed, transitions can only occur between state with values of l that diff:lsi;a u5b (s pgszo4)pk,rz*er by
A. four units.
B. three units.
C. two units.
D. one unit.
