3. The difference between cysteine and homocysteine is the same as between serine and homoserine. This change from the common amino acid is:
(A) one additional carboxyl group
(B) one additional methylene (-CH2-) bridge
(C) two additional amine groups
(D) presence of a ring system

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統計： A(5), B(35), C(0), D(2), E(0) #3094117