In a teaching session, Agonist is correctly explained as:

A. A ligand that binds to a receptor and activates it to produce a biological response. B. A reversible receptor blocker whose effect can be overcome by increasing agonist concentration. C. Presystemic metabolism that reduces oral bioavailability before a drug reaches systemic circulation. D. A drug class that antagonises beta-adrenergic receptors and can reduce heart rate.