Task: Question Answering
Input (English): Question: The primary function of interleukin 2 (IL-2) is: 
 Options:  <1> Induce activation of macrophages. <2> Promote isotype changes in immunoglobulins produced by activated B cells. <3> Induce the synthesis of acute phase proteins by hepatocytes. <4> Promote the proliferation of T lymphocytes <5> Promote the proliferation of B lymphocytes
Output (English): 4