Which of the following affects blood volume?
select one:
a. the speed at which the blood flows
b. the strength at which the heart contracts
c. the amount of water present in the blood
d. the rate at which the heart contracts

The correct answer would be c. The amount of water present in the blood  

Blood volume refers to the volume of the blood present in the circulatory system of an individual.

For example, in a typical human adult, the blood volume is approximately 5.1 liters.

Water plays an important role in determining the volume of the blood in an organism.

In humans, blood volume is regulated with the help of kidneys. When blood volume is higher than the optimum value, the kidneys excrete more amount of sodium and water in the urine whereas when it is lower than the optimum then kidneys retain more amount of sodium and water in the body.

protist eukaryotes that are not plants, animals, or fungi ("misfits")

thinking it would be d not too sure but i think it's right