The atria begin to contract following

A. depolarization of the atria and pump blood into the ventricles.
B. When the pressure falls below that of the atria, blood moves from the atria into the ventricles, opening the atrioventricular valves and marking one complete heart cycle.
C. The ventricles begin to contract, raising pressure within the ventricles.

A. depolarization of the atria and pump blood into the ventricles.

Explanation:

Contraction of atria is also called atrial systole which is responsible for the pumping of blood into the ventricles. The blood moves from the atria into the ventricles, opening the atrioventricular valves and completed one heart cycle. Contraction of ventricle is known as ventricle systole. During contraction of ventricle, blood is transported to two regions, one is the lung for cleaning and other is all parts of the body.

you must remain stopped until the stop signal is retracted and the red lights are turned off. approaching traffic in the opposite roadway of a divided highway shall not be required to stop, even if the school bus has the stop-arm extended and the alternately flashing warning signal lamps on.

"a.) well–developed vascular system."