What is a positive effect that predators have on a prey population? a. they eliminate the prey population in order to maintain competition over resources such as food and water. b. prey are able to increase their populations and use of food, shelter, and water when predators are on the hunt. c. predators maintain an unchanged prey population to increase the use of food, water, and shelter. d. they keep the prey population low so they will be less likely to deplete resources such as food, water, and shelter.

The kidney filters potentially toxic substances in the blood, and thus “clears” the blood of those substances. this clearance function is dependent upon and proportional to the diffusion gradient of the substance across filtering capillaries, i.e. if the concentration of the substance is doubled, twice as much will be cleared from each ml of blood that is filtered. suppose that the body produces a constant amount of a substance x per unit of time. the kidneys eliminate substance x at a rate directly proportional to the concentration of the substance and the volume of blood cleared each minute (c): elimination = c × [x], where [x] is the steady-state concentration of substance x. imagine an individual with an initial concentration of x equal to [x]0 who develops kidney disease. her baseline clearance c0 drops to one half of the original (½c0). what is the new steady state concentration of x? (for simplicity, assume that substance x is 100% filtered by the kidney).