The base adenine pairs with thymine, and the base cytosine pairs with guanine.
Explanation:
From Chargaff's discovery, which was that the DNA of any given species contains equal amounts of adenine and thymine, and equal amounts of cytosine and guanine, we can deduce that if there is an equal amount of two bases in DNA, those two bases must pair together. From this information, we can then go on to say that since there are equal amounts of adenine and thymine, they pair together, and since there are equal amounts of cytosine and guanine, they also pair together.
