Answer:
A always pairs with T and C always pairs with G. A and G are purines, meaning that they have two rings, while T and C are pyrimidines, meaning they have one ring. Due to the Watson and Crick model, the width of DNA is constant, meaning that there must be a consistant width in each base pairing. As a result, only a purine can pair with a pyrimidine, giving each base pairing a total of 3 rings.
Explanation:
