When you touch an object that is very hot you quickly pull your hand back. What is the correct pathway of nerve impulses for this behavior?
A. Heat receptors in the hand, motor neurons, hand muscles, sensory neurons, brain.
B. Hand muscles, motor neurons, brain, sensory neurons, heat receptors in hand.
C. Brain, sensory neurons, heat receptors in hand, motor neurons, hand muscles.
D. Heat receptors in the hand, sensory neurons, brain, motor neurons, hand muscles.

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).