Which of the following is TRUE regarding the flow of blood through the heart: a) The pulmonary artery carries deoxygenated blood.
b) The aorta is responsible for carrying oxygenated blood to the pulmonary system.
c) During systole, blood is going from the atria to the ventricles by gravity.
d) During diastole, blood is flowing over the semilunar valves

a The pulmonary artery carries deoxygenated blood.

Explanation:

This is the the only artery that transports deoxygented blood in mammals. specifically it carry deoxgenated blood from the the atria  to the pulmonary circulation. That is to the lungs  for the exchange of the C02 in the deoxygenated blood with oxygen  in the lungs.

The  blood thus become oxygenated,and therefore return through the pulmonary veins to the left ventricle.

From the left ventricle ,through ventricular systole,blood is empty into the aorta through the aortic valves into the  systemic circulation.

Although,they carry blood away from the heart,but the blood is de oxygenated instead of the usual oxygenated by arteries,They are still regarded as arteries  because of convey of blood away from the heart,a typical role of arteries

many other transport proteins, often called pumps, move materials against a concentration gradient. active transport drives molecules across a membrane from a region of lower concentration to a region of higher concentration. this process, shown in figure 5.1, uses transport proteins powered by chemi- cal energy.