2. Adenine and guanine are double-ringed purines

single-ringed purines

double-ringed pyrimidines

D. single-ringed pyrimidines

amino acids.

single-ringed purines

Explanation:

Adenine and guanine are purine bases. They are structures composed of a 5-sided and 6-sided ring. Cytosine and thymine are pyrimidines which are structures composed of a single 6-sided ring. Adenine binds to thymine, while cytosine binds to guanine. This relationship is a form of complementary base pairing. These complementary bases are bonded together by hydrogen bonds.