Why Would Drugs That Are Allosteric Modulators Be Preferable to Agonists

Why would drugs that are allosteric modulators be preferable to agonists or antagonist drugs in the treatment of psychiatric disorders, such as schizophrenia?

A) Allosteric modulators often have fewer side effects because they show greater receptor subtype selectivity.
B) Allosteric modulators have reduced receptor subtype selectivity, so they treat a wider range of disorders.
C) Allosteric modulators are readily available; agonists and antagonists are more difficult to synthesize.
D) Allosteric modulators consistently lead to release of more transmitter, thereby more effectively treating a disorder.