Shortly after administering a second dose of 4 mg of morphine to a 49-year-old woman who is experiencing chest pain, the patient's level of consciousness markedly decreases. Further assessment reveals that she is hypotensive, bradycardic, and hypoventilating. You should:

A. immediately intubate her to protect her airway.
B. elevate her legs and give a 500-mL saline bolus.
C. administer 0.5 mg of atropine and reassess her.
D. assist her ventilations and administer naloxone.