3. When the divalent antibody was made monovalent by separating the antigen-binding domains (Fabs) from the constant domain (F), the Fab portion, but not the F. portion, bound the EGF receptor. Neither portion triggered cell proliferation. Which of the following conclusions is best supported by these results? (A) EGF binds a divalent antibody but not a monovalent antibody. (B) Signal transduction requires EGF or a monovalent antibody. (C) Binding of the Fab portion alone leads to internalization of the receptor. (D) Only a divalent antibody recognizes the binding site for EGF. (E) Dimerization of EGF receptors by a divalent antibody leads to signal transduction.