[单选题]
According to the selection rule, when a*gjr5w7*)ktg7, jps , oc6 xdfiy bc*t photon is emitted orn;+4qoj 9k f02i)c;v j3 ouqstrs.lal absorbed, transitions can o 4 nl.j)9 s3;oo u+fqri;2katc js0qvlnly occur between state with values of l that differ by
A. four units.
B. three units.
C. two units.
D. one unit.
