Thymine and cytosine differ from adenine and guanine in that ________.

A)thymine and cytosine are single-ring structures, whereas adenine and guanine are double-ring structures
B)thymine and cytosine are only found in DNA, whereas adenine and guanine are found in both DNA and RNA
C)thymine and cytosine are only found in DNA, whereas adenine and guanine are only found in RNA