[单选题]
According to the selection ru h f/cs qxnt:1l )/fh-cxsow.8le, when a photon is emitted or a neob+d;. q.vn6z lh1tbsorbed, transitions can only occur between state with values of l oqne.t+.n1v; d lzh6b that differ by
A. four units.
B. three units.
C. two units.
D. one unit.
