The endosymbiotic theory proposes that mitochondria in eukaryotic cells arose from

The endosymbiotic theory proposes that mitochondria in eukaryotic cells arose from a prokaryote that lived in symbiosis with a eukaryotic cell.

Explanation:

The endosymbiotic theory was proposed by Lynn Margulis in 1981 and admits that some organelles (mitochondria and chloroplasts) in eukaryotic cells arose thanks to a symbiotic association.  It is believed that mitochondria and chloroplasts are descendants of prokaryotic organisms that were captured and adopted by some cell, thus living in symbiosis.

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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