The purines in DNA are

A.thymine and cytosine
B.thymine and guanine
C.adenine and guanine
D.adenine and cytosine

Adenine and guanine are purines. Thymine and cytosine are pyrimidines.

The correct answer is C. adenine and guanine.

To arrive at this answer, you need to understand the structure of DNA and its base pairs. DNA, or deoxyribonucleic acid, is composed of nucleotides. Each nucleotide consists of three components: a sugar molecule called deoxyribose, a phosphate group, and a nitrogenous base.

There are four types of nitrogenous bases found in DNA: adenine (A), thymine (T), guanine (G), and cytosine (C). Adenine and guanine are classified as purine bases, while thymine and cytosine are classified as pyrimidine bases.

In DNA, the bases always pair up in a specific way: adenine pairs with thymine (A-T) and guanine pairs with cytosine (G-C). These base pairs form the rungs or steps of the DNA ladder-like structure, known as the double helix. The pairing of A-T and G-C is due to hydrogen bonding between the bases; adenine always pairs with thymine via two hydrogen bonds, while guanine always pairs with cytosine through three hydrogen bonds.

Based on this pairing rule, we can determine that adenine will always be paired with thymine, which means cytosine cannot be a purine base. So, the correct answer is C. adenine and guanine.