I think it would be to exchange gases. When you think about it, plants in the past were always underwater. They didn't have any air. Now, they are on land and need to be able to exchange gases from the leaves to the roots.
A constant variable would be an element that remains unchanged and constant throughout the course of the experiment / investigation.
B
Because virus do contain genetic material and duh they make people sick and yes they are made of more than one cell
Answer:
D. Control the blood flow into the capillary bed
Explanation:
The degree of filling of the capillary bed and level of arterial pressure within the vascular system is determined mainly by the degree of tonus (firmness) in the smooth muscle of the arterial wall. Arterioles have relatively narrow lumina, thick muscular wall and precapillary sphincter.