Correct Answer: A
Explanation:
: (a) The bases are of two types-purines and pyrimidines. The purine derivatives adenine (A) and guanine (G) are double ring structures whereas pyrimidine derivatives thymine, cytosine and uracil are single ring structures. Thymine (T) and cytosine (C) are found in DNA and cytosine (C) and uracil (U) is found in RNA. In the given question there are 4 pyrimidines as 2 cytosine and 2 thymine in optional.