The answer should be C, the heart and the blood vessels.
The heart acts as a pump in the circulatory system. It pumps blood to organs and tissues all around the body. Meanwhile, the blood vessels (arteries, veins, and capillaries) help transport the blood to tissue and cells and back to the heart. Each type of blood vessel has their own function.
The brain is not involved in this system, as the whole circulation process does not require any us to make any decision.
In addition, there's also one structure that is in the blood vessels, but technically not another item for the circulatory system. That's valves, they are like flaps that ensure blood flowing in one wag direction and to prevent the back flow of blood.
So, the answer is C.
