The upper chambers of the heart are called , and the lower chambers of the heart are called .

The upper chamber is called an atrium, or auricle, and the lower chamber is called a ventricle. The two atria act as receiving chambers for blood entering the heart; the more muscular ventricles pump the blood out of the heart.

The pumping of the heart involves the alternate contraction and relaxation of the chambers of the heart. The upper chambers of the heart are called  atria , and the lower chambers of the heart are called  ventricles .

the answer is:

carbon dioxide

i believe your answer would be c.. : )