Task: Question Answering
Input (English): Question: What is a prodrug? 
 Options:  <1> A drug of series of a therapeutic class. <2> A drug that has been withdrawn from the market. <3> An inactive product that is enzymatically converted to an active drug. <4> A compound of natural origin from which one or more drugs are obtained by hemisynthesis. <5> A drug that lacks activity but enhances the action of others.
Output (English): 3