Thymine Structure and Classification Steps:

• Recall thymine as a nitrogenous base in DNA, one of four: adenine, guanine, cytosine, thymine.
• Classify bases: purines (adenine, guanine) have two fused rings; pyrimidines (cytosine, thymine) have one ring.
• Confirm thymine's structure: single six-membered heterocyclic ring with keto groups.
• Check base pairing: thymine pairs with adenine via 2 hydrogen bonds, unlike guanine-cytosine's 3.

Why A is correct:

• Thymine possesses a single-ring pyrimidine structure, matching the definition of pyrimidines as monocyclic aromatic heterocycles.

Why the others are wrong:

• B: True but incomplete; omits pyrimidine classification essential for base identity.
• C: True but incomplete; lacks structural detail of the single ring.
• D: False; thymine-adenine pairing involves 2 hydrogen bonds, per Watson-Crick model.

Final answer: A