19. When an hydrogen electron makes transition from n = 2 to n = 1, which of thefollowing statements is (are) true?
I. Energy is emitted.
II. Energy is absorbed.
III. The electron loses energy.
IV. The electron gains energy.
V. The electron cannot make this transition.
(A) II, III
(B) V
(C) I, III
(D) II, III

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