The activation of beta2 receptors in the bronchi causes bronchodilation (an increase in diameter of the bronchi), making it easier to breathe. Which of the following neurotransmitters would be useful for a person suffering from an episode of bronchoconstriction (a reduction in the diameter of the bronchi)?

a. acetylcholine
b. epinephrine
c. vasopressin (or ADH)
d. oxytocin