Answer:
A ) adenine and thymine
Step-by-step explanation:
DNA has two types of nitrogenous bases: purines and pyrimidines. Adenine and guanine are purines while cytosine and thymine are pyrimidines.
A purine always pairs with a pyrimidine. Hence, adenine in DNA pairs with thymine while guanine pairs with cytosine. This is done to maintain the constant width of the double helix throughout the length.