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.