[单选题]
According to the selection rule, when a photkp iwf z+lk1jh l )/u:vr0e7z.ev67ryon 1j-p- ej)afgn is emitted or absorbed, transitions can only occur between state with values of l thap-nf g- )j1jeat differ by
A. four units.
B. three units.
C. two units.
D. one unit.
