Which of the following molecular mechanisms is NOT used in the cell in order to produce abrupt responses to a signal?

A) Activation of a protein only when bound to multiple molecules of an activating ligand
B) Activation of a protein only when it is phosphorylated at multiple sites
C) Activation of a protein by simultaneously activating its activator(s) and inactivating its inhibitors
D) A positive feedback loop in which an activated protein positively regulates its activator
E) A negative feedback loop in which an activated protein inhibits its activator without delay