Which of the following distinguishes a prokaryotic cell from a eukaryotic cell ? the presence of a nucleus, the presence of a cell membrane, the ability to interact the environment, the ability to reproduce ?

Only eukaryotes have a nucleus...all the other answers are true for both euks and proks...remember that only eukaryotes have internal membranes.

c genetic engineering

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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