[单选题]
According to the selection2-tcyzh+*p k qbc1sz1 rule, whs5p,y uf)dj 4zen a photon is emitted or absorbed, transitions can only occur between state with values of l ths 4ufpzd ,)yj5at differ by
A. four units.
B. three units.
C. two units.
D. one unit.