19. All are important functional groups participating in H-bond formation EXCEPT: (A)amino group of the cytosine, adenine and guanine. (B) the ring nitrogens at position 3 of pyrimidines and 1 of purines. (C) the electronegative oxygen atoms. (D) the electron density in the double bonds of the bases. (E) the carbonyl group at position 4 of uracil and thymine, position 2 of cytosine, and position 6 of guanine.