Which of the following statements is true? (a) extracellular signal molecules that are hydrophilic must bind to a cell-surface receptor so as to signal a target cell to change its behavior. (b) to function, all extracellular signal molecules must be transported by their receptor across the plasma membrane into the cytosol. (c) a cell-surface receptor capable of binding only one type of signal molecule can mediate only one kind of cell response. (d) any foreign substance that binds to a receptor for a normal signal molecule will always induce the same response that is produced by that signal molecule on the same cell type.