I would love to help answer, but i think i need a little more context here.
Answer:
muscles -D
Explanation:
the circulatory system consists of three independent systems that work together the heart, the lungs, and arteries, veins , coronary and portal vessels .
Okay, this is an interesting one but mainly no, most activity's will so you don't push yourself but if you really wanted to you could go for however long you want as long as you are not doing harm to your body, depending if you are on a fitness program there will be limits.
I’m gonna go with the third option as it sound the most responsible
