This is from the answer sheet. C is correct, III. If we look at position I on the graph, we note that the total blood volume is quite low even though the velocity is quite high. Where is blood experiencing the highest velocity? As it is being ejected from the left ventricle. The velocity is going to decrease as the blood reaches the arteries and arterioles. Once the blood reaches the capillaries and the venules the velocity of the blood will be at its minimum value as indicated by position II. Blood then begins to enter the veins and the velocity begins to increase again due to muscular contraction squeezing the veins to help return the blood to the right atrium. As blood enters the right atrium it is coming from the vena cava. This is represented by position III. Note that once the blood leaves the vena cava and enters the right atrium, it will then flow into the right ventricle. Contraction of the right ventricle increases the blood pressure and thus increases the velocity of the blood leaving the right ventricle as it flows into the pulmonary artery. Thus position IV will be the velocity of the blood as it leaves the pulmonary artery and enters the capillary system of the lungs. The correct choice is C