Task: Question Answering
Input (English): Question: The so-called poly-Ig receptor: 
 Options:  <1> Facilitates the transport of IgA through the mucosal epithelium. <2> Mediates the transport of IgG through the placenta. <3> Stabilizes the polymer structure of an IgM. <4> It is expressed on the surface of B lymphocytes. <5> It is expressed on the surface of tissue macrophages.
Output (English): 1