In a teaching session, Proton-pump inhibitor is correctly explained as:

A. A drug that irreversibly suppresses the gastric hydrogen-potassium ATPase. B. A muscarinic receptor antagonist that can increase heart rate and reduce secretions. C. A ligand that binds to a receptor and activates it to produce a biological response. D. A drug class that antagonises beta-adrenergic receptors and can reduce heart rate.