How can DNA bases pair up

via hydrogen bonds; there are two between adenine and thymine and three between cytosine and guanine

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 believe it is "they are not used up by reactions" as the answer to your question, text me back if this is correct because i just learned this.