Which one of the following is an incorrect combination?
(a) Adenine, thymine, cytidine
(b) Adenine, cytosine, thymine
(c) Guanine, thymine, uracil
(d) Cytosine, uracil, guanine
(a) Adenine (A), guanine(G), cytosine (C), thymine (T) and uracil (U) are all nitrogenous bases, whereas cytidine is a nucleoside i.e, a combination of a nitrogenous base (cytosine) with a pentose sugar (ribose).