Answer:
True
Explanation:
Unlike arteries, veins contain valves that ensure blood flows in only one direction. (Arteries don't require valves because pressure from the heart is so strong that blood is only able to flow in one direction.) Valves also help blood travel back to the heart against the force of gravity.
All t cells originate from haematopoietic stem cells in the bone marrow
I am going to have to say answer C is correct.
It could be physical activity
