Task: Question Answering
Input (English): Question: Which of the following antitumor agents acts forming a covalent and irreversible complex with the thymidylate synthase and its cofactor, previous activation to the corresponding deoxynucleotide? 
 Options:  <1> Methotrexate <2> Bortezomib. <3> Cyclophosphamide <4> 5-Fluorouracil. <5> Taxol.
Output (English): 4