In which situation would an injected drug be the most effective agonist for an endogenous chemical messenger?

A)The drug has a lower affinity for the messenger's receptors than the messenger does.
B)The drug achieves 50% saturation of the messenger's receptors at a lower concentration than that required by the messenger.
C)The drug does not couple to the binding site of the messenger's receptor.
D)The drug binds to an alternate binding site on the protein receptor and reduces its affinity for the endogenous chemical messenger.