Pyrimidines are single-ring nitrogenous bases in DNA and RNA

Steps:

• Recall purines have a double-ring structure (adenine, guanine); pyrimidines have a single-ring structure (cytosine, thymine, uracil).
• Identify thymine (in DNA) and uracil (in RNA) as pyrimidines.
• Eliminate options mixing purines and pyrimidines.
• Select the option listing only pyrimidines.

Why C is correct:

• Thymine and uracil both possess the single six-membered pyrimidine ring structure, distinguishing them from purines.

Why the others are wrong:

• A: Adenine is a purine (double ring); thymine is a pyrimidine.
• B: Guanine is a purine (double ring); cytosine is a pyrimidine.
• D: Guanine is a purine (double ring); uracil is a pyrimidine.

Final answer: C