The pairing of nitrogenous bases in DNA is specific because which feature ensures that adenine pairs with thymine and guanine pairs with cytosine?

• A. Hydrogen-bonding patterns and complementary base shapes allow only specific purine–pyrimidine pairs to fit the helix geometry
• B. All nitrogenous bases form the same number of hydrogen bonds, so any base can pair with any other base equally well
• C. The phosphate groups on opposite strands attract each other and force specific base pairing
• D. Covalent bonds between bases determine pairing specificity by permanently linking A to T and G to C

Answer: (A)