a
Thus, in \(DNA\), the purines adenine \((A)\) and guanine \((G)\) pair up with the pyrimidines thymine \((T)\) and cytosine \((C)\), respectively. In \(RNA\), the complement of adenine \((A)\) is uracil \((U)\) instead of thymine \((T)\), so the pairs that form are adenine:uracil and guanine:cytosine.