Definition: In this task, you are given a multiple-choice question about healthcare. Answer the question based on your information and classify your answers into '1', '2', '3', and '4'.
Input: Question: Regarding enzymatic cofactors:
Options: <1> All enzymes are required to be active. <2> Its binding to the enzyme is always covalent. <3> There are no two enzymes that use the same cofactor. <4> They can be metal ions. <5> They only have structural function.
Output:
4