The Mechanism of Action That Differentiates the Opioid Agonists from the Opioid

The mechanism of action that differentiates the opioid agonists from the opioid agonist-antagonists is:

A) Opioid agonists activate two receptors, the opioid agonist-antagonists only occupy one receptor.
B) Opioid agonists block one receptor, the opioid agonist-antagonists block both receptors.
C) Opioid agonists activate one receptor, the opioid agonist-antagonists activate both receptors.
D) Opioid agonists block two receptors, the opioid agonist-antagonists have no effect on any receptors.