Task: Question Answering
Input (English): Question: The simultaneous expression of IgM and IgD as membrane immunoglobulins occurs in: 
 Options:  <1> B cells that respond to thymoindependent antigens. <2> B cells that respond to thymus-dependent antigens. <3> Activated B cells stimulated by Interleukin 4 (IL-4). <4> Immature virgin B cells. <5> Mature virgin B cells.
Output (English): 5