19. When a carbonyl is part of a conjugated π-network, the C=O stretch:
(A) has a higher frequency than in a nonconjugated system.
(B) has a lower frequency than in a nonconjugated system.
(C) always occurs at 1710 cm-1 .
(D) occurs around 2700 cm-1 .
(E) cannot be distinguished from the C=O stretch in a nonconjugated system.

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統計： A(1), B(6), C(3), D(0), E(0) #1074565