The reason that an action potential is so brief is that

The reason that an action potential is so brief is that
  a. a wave of repolarization immediately follows the
  rising portion of the spike.
  b. the closing of potassium gates allows the voltage
  difference across the neural membrane to be
  restored.
  c. the channels for sodium movement remain open.
  d. the sodium-potassium pump restores the
  electrical gradients.
  e. the membrane limits electrical activity.

Question 2

Once a threshold is reached,
  a. the number of sodium gates that open depends
  upon the strength of a stimulus and its duration.
  b. a graded local potential will be unable to spread
  to a trigger zone of the nerve membrane.
  c. the resting potential of a neuron is restored.
  d. potassium channels in the input zone open.
  e. progressive opening of more and more sodium
  gates and the accompanying flow of sodium ions
  is an example of positive feedback.