Which label belongs in the area marked X?

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

1. efficient use of space

2. elimination of groundwater contamination

3. energy generation

4. lower carbon footprint