2. Which is not correct concerning the models that are accepted to describe cooperative binding? (A) In the sequential model, the binding of a ligand changes the conformation of the subunit to which it binds, which in turn induces a change in neighboring subunits. (B) All known allosteric proteins exhibit either the concerted or sequential model exclusive of the other. (C) Both models incorporate a low affinity T-state and a higher affinity R-state. (D) Both models explain the sigmoid-shaped binding curve. (E) In the concerted model, all molecules exist either in the T-state or the R-state.