Concept:DNA base pairing follows the rule that a purine always pairs with a pyrimidine.Explanation:Adenine is a purine base, while thymine is a pyrimidine base.They pair through hydrogen bonds, giving the DNA double helix a consistent diameter.Adenine cannot pair with another purine because two purines would make the helix too wide.Options A, B, and C do not explain the specificity of adenine-thymine pairing.Answer:D. one is a purine base and the other a pyrimidine.