In a dna double helix an adenine of one strand always pairs with a(n) of the complementary strand, and a guanine of one strand always pairs with a(n) of the complementary strand. a) cytosine thymineb) uracil cytosinec) guanine adenined) thymine cytosinee) cytosine uracil

d) thymine ... cytosine

Explanation:

DNA has two different types of nitrogenous bases. Two bases of DNA are purines and rest two are pyrimidines. The purine bases are DNA are adenine and guanine. Each of these two bases has a large structure with two rings. On the other hand, thymine and cytosine are the pyrimidines. They have a relatively smaller single ring structure.

To maintain the constant distance between the two strands of a helix, a purine always pairs with a pyrimidine. Therefore, adenine makes two hydrogen bonds with thymine and pairs with it.  The guanine base of DNA makes three hydrogen bonds with cytosine of the other strand.