Thymine and cytosine differ from adenine and guanine in that

The correct answer is: thymine and cytosine are single-ring structures called pyrimidines.

Explanation:

Thymine, Cytosine, Adenine and Guanine are the nitrogenous bases that form the four nucleotides in DNA strands. In RNA strands, there's no thymine but there's uracil.

These bases have the function to be the fundamental units of the genetic code.

The structure of these nitrogenous bases consists of a skeleton with the shape of one ring (which grants them the name of pyrimidines) or two fused rings (these are called purines). Thymine and Cytosine are pyrimidines, while Adenine and Guanine are purines.