Complementary base-pairing rules simply state that
A. either purine can bond with either complementary pyrimidine. For example, adenine can pair with cytosine or thymine.
B. base pairs form between complements. For example, adenine pairs with guanine, and thymine pairs with cytosine.
C. like bases complement like bases. For example, adenine pairs with adenine, guanine with guanine, etc.
D. base pairs form between complements. For example, adenine pairs with thymine, and cytosine pairs with guanine.