Which one of the following enzymes of nucleotide metabolism is correctly paired with its pharmacologic inhibitor?

A. Dihydrofolate reductase—methotrexate

B. Inosine monophosphate dehydrogenase—hydroxyurea

C. Ribonucleotide reductase—5-fluorouracil

D. Thymidylate synthase—allopurinol

E. Xanthine oxidase—probenecid