Voltage-gated channels contain charged protein domains, which are sensitive to changes in membrane ...

Voltage-gated channels contain charged protein domains, which are sensitive to changes in membrane potential. By responding to a threshold in the membrane potential, these voltage sensors trigger the opening of the channels.
 
  Which of the following best describes the behavior of a population of channels exposed to such a threshold?
  (a) Some channels remain closed and some open completely.
  (b) All channels open completely.
  (c) All channels open partly, to the same degree.
  (d) All channels open partly, each to a different degree.