The switch of one B cell from producing one class of antibody to another antibody class that is responsive to the same antigen is due to A) mutation in the genes of that B cell, induced by exposure to the antigen. B) the rearrangement of V region genes in that clone of responsive B cells. C) a switch in the kind of antigen-presenting cell that is involved in the immune response. D) a patient’s reaction to the first kind of antibody made by the plasma cells. E) the rearrangement of immunoglobulin heavy-chain C region DNA.