In one sentence explain why carbon dioxide is considered the most important greenhouse gas, although it traps much less heat than other greenhouse gases such as methane, nitrous oxide and CFCs.

Because plants need carbon dioxide more than any other compound/element. Also, heat doesn't make much of a difference in a plants growth or production. It's sunlight that makes a difference.

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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a. to encourage apoptosis