Acountercurrent heat exchanger enables an animal to
a. absorb heat from the environment
b. produce more heat when needed
c. reduce the loss of heat to the environment
d. increase heat loss by evaporation e. slow metabolism when food is not available

the answer that best describes the relative refractory period in the neuron is the resting membrane potential is restored, but concentrations of sodium and potassium are not restored to their original state.

the refractory period is the time during which another stimulus given to the neuron will not lead to a second potential. thus, because na+ channels are inactivated during this time, additional depolarizing stimuli do not lead to new action potentials.  

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if the type of the food available changes, then the frequency of beak types will change, because birds with beaks more suited to the available food will be more successful over time.