What does Chargaff's rule state about the base composition of DNA?

The amount of purines equals the amount of pyrimidines.

The amount of adenine equals the amount of thymine, and the amount of guanine equals the amount of cytosine.

The amount of adenine equals the amount of cytosine, and the amount of guanine equals the amount of thymine.

The amount of adenine equals the amount of guanine, and the amount of thymine equals the amount of cytosine.