Which form successively larger vessels that carry blood toward the heart?

Veins.

Explanation:

Heart is the main organs of circulatory system that transport blood in the body.Human heart is four chambered and separates the oxygenated and deoxygenated blood in the body.  

Arteries are responsible for the passage of blood from heart to the body parts. The deoxygenated blood from the body collect in the veins and transfer to heart. Arteries has high pressure than veins and veins are thinner than artery.

Thus, the answer is veins.

hydrogen is a compound or a single element and a element is a pure substence that cannot be separated into simpler subtances

clorine is a compound its a chemical substance that forms when two or more different elements combine

so both hydrogen and clorine are compounds