Which bases are classified as pyrimidines?

Pyrimidines are one of the two categories of nitrogenous bases found in nucleic acids, the other category being purines. The pyrimidine bases are characterized by their single-ring structure. In DNA and RNA, the pyrimidines include cytosine (C), thymine (T), and uracil (U).

Option B correctly identifies thymine (T) and cytosine (C) as pyrimidines. Thymine is found in DNA, while uracil takes the place of thymine in RNA. This distinction is important as it highlights the functional roles these bases play in the genetic code.

The other options list combinations of purines and pyrimidines incorrectly. For instance, adenine (A) and guanine (G) are purines and do not belong in the same classification as pyrimidines. Recognizing the specific structures and classifications of nitrogenous bases is essential for understanding the structure and function of nucleic acids in biology.