Shown below (left) is a schematic diagram representing the conformational equilibrium of a heteromeric allosteric enzyme. Note that the R state has higher catalytic activity than the T state.

Also shown below (right) are the kinetics curves for this enzyme alone and in the presence of an allosteric modulator.

Based on the kinetics curves, which site on the enzyme (A-D) is this modulator likely to bind to most strongly?