You are trying to ventilate an 18-month-old baby who has stopped breathing. You have laid him down on a flat surface to ventilate him, but you are unable to get his chest to rise. You should:
A. suction his mouth to clear secretions.
B. flex his neck toward his chest.
C. place a folded towel under his shoulders.
D. tip his head back farther to open his airway.