Which of the following structures does NOT participate in H-bond formation?

A)the amino group of cytosine,adenine,and guanine
B)the ring nitrogens at position 3 of pyrimidines and 1 of purines
C)the carbon double bonds found in the bases
D)the carbonyl group at position 4 of uracil and thymine,position 2 of cytosine,and position 6 of guanine