Which of these statements best describes a scientific theory?
It is a description of an observation which does not change over time.
It is a description of an observation after research over a period of time.
It is an explanation for an observation which is not based on data.
It is an explanation for an observation after several valid experiments.

It is an explanation for an observation after several valid experiments.

Explanation:

The definition of a scientific theory is an explanation of an aspect of the natural world that can be repeatedly tested and verified in accordance with the scientific method, so the closest one is that it is an explanation for an observation after several valid experiments.

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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