4. Which of the following accounts for the extent of proliferation observed in the retrovirus-infected cells not treated with growth factors? (A) The v-erbB product has a function similar to that of activated EGF receptor. (B) EGF binds to both the EGF receptor and the v-erbB product. (C) The v-erbB product activates both the LCGF and the EGF receptors. (D) The v-erbB product antagonizes the action of the EGF receptor. (E) The v-erbB product induces secretion of EGF and LCGF.