Anyone know what the answer is ?

In this experiment, the was intentionally manipulated. it is the independent variable. the dependent variables that were measured were the

Veins have a much lower blood pressure than arteries. which of these prevents backflow of blood in veins? a. pressure applied by the heart b. one–way valves in veins c. thin muscular walls of veins

Why do mountaineers become breathless as they reach high altitudes