To help control Chloe's nausea, Chloe's doctor gives her Zofran, which blocks the action of serotonin, a chemical involved in the body's nausea response. In Chloe's case, the medication prescribed by her doctor acts as an

• A. excitatory neurotransmitter.
• B. inhibitory neurotransmitter.
• C. antagonist.
• D. agonist.