a scientist is studying trait variation in oldfield mice. mice that live on the beach are lighter colored than mice that live inland. she collects infant mice from populations in both regions and rears them under two conditions in her lab: on sand and on soil. reared on sand, the beach mice are just as light as their parents, while the inland mice are just as dark as their parents. reared on soil, both types of mice are dark. from this she concludes that:

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).