Increasing the amount of action potentials that arrive at the presynaptic terminal will:

a) increase the amount of depolarization in the terminal
b) increase the amount of ca2+ that enters the presynaptic terminal.
c) increase the number of vesicles that fuse with the presynaptic membrane
d) increase the amount of neurotransmitter released.

Increasing amount of action potentials that arrive at pre synaptic terminal will increase the amount of Ca2+ that enters the presynaptic terminal.

Explanation:

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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when atoms of different types of elements join together, they make molecules called compounds. water consists of compound molecules made up of 2 hydrogen atoms and 1 oxygen atom. this is why it's called h2o. water will always have 2 times the number of hydrogen atoms as oxygen atoms.