30. In the emission spectrum of mercury, there is a spectral line at 404.66 nm produced by mercury atoms undergoing electronic transitions from n=7 to n=6, where n is the principal quantum number. Since the upper level is threefold degenerate, in an external magnetic field the symmetry is broken and the degeneracy disappears, and thus the spectral line splits into three, as shown in the figure. This spectroscopic phenomenon is ascribed to

(A) Meissner effect
(B) Stark effect
(C) Raman effect
(D) Hall effect
(E) Zeeman effect. 。