Answer:
a
Explanation:
generally there are four types of cell signaling. this a common mcq which was asked several times.
Arteries carry blood (with oxygen) from our heart out to your body, while veins carry blood (without oxygen) from your body to your heart. Arteries are strong, flexible blood vessels that are able to expand (get bigger) and contract (get smaller). They expand as your heart beats, and contract between heart beats. Veins are less flexible than arteries. They have valves (doors) that open and close to let blood through.
