Concentration:
the amount of a substance within a of another
substance

Volume

Explanation:

In chemistry, the CONCENTRATION of a solution is the amount of a substance present within a volume of another substance. In other words, the concentration refers to the amount of a solute dissolved in a particular volume of a solvent to make a solution.

Concentration can be expressed in mass per volume i.e. g/cm³ or mol per volume i.e. mol/L or mol/dm³ etc. The amount of a substance is measured in Mass or mole, while the volume of a substance in Litres (L).

the most logical answer would be a

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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