Answer:
Watson and Crick concluded that a purine base always pairs with a pyrimidine base.
Step-by-step explanation:
DNA nucleotides have two types of nitrogenous bases: purines and pyrimidines. Adenine and guanine are purine bases while thymine and cytosine are the pyrimidine bases. To have a double helix with a constant distance between the two DNA strands, a purine base with a two-ring structure always pairs with a pyrimidine base with a single ring structure. In this way, the distance between the two DNA strands is always constant. Therefore, they concluded that adenine (a purine) pairs with thymine (pyrimidine) while guanine (purine) pairs with cytosine (pyrimidine).