4. What is the most likely function of the structure labeled X in the figure below?

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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i would go with b

cytokinesis occurs after telophase so that eliminates d

and the entire reason for cytokinesis is for cellular reproduction

the tricuspid valve is present between right atrium and right ventricle