Task: Question Answering
Input (English): Question: The 2,4-diaminopteridine systems, such as aminopterin or methotrexate, are good antitumor agents since they reduce the biosynthesis of purines and pyrimidines by inhibiting the enzyme: 
 Options:  <1> Timidylate synthetase <2> Aromatase <3> Topoisomerase <4> Reverse transcriptase <5> Dihydrofolate reductase.
Output (English): 5