Task: Question Answering
Input (English): Question: In the Th2-B cooperation process, what does the Th2 cell recognize on the surface of the B cell? 
 Options:  <1> The antigen bound to a surface immunoglobulin. <2> A fragment of antigen bound to the CD21 molecule. <3> Several cross-linked antigen fragments. <4> A fragment of antigen bound to a class II histocompatibility molecule. <5> A fragment of antigen bound to a class I histocompatibility molecule.
Output (English): 4