Which electron carrier is used in the redox reactions in cellular respiration?

The right answer is NAD+ (NADH).

NAD (Nicotinamide-adenine-dinucleotide) is a coenzyme found in cells. It is a dinucleotide, insofar as the molecule consists of a first nucleotide, whose nucleic base is adenine, united to a second nucleotide, the base of which is nicotinamide. NAD exists in an oxidized form, denoted NAD+, and a reduced form, denoted NADH.

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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