When an action potential reaches an axon terminal, calcium

When an action potential reaches an axon terminal, calcium
 
  a. rushes into the cell immediately, because this negatively charged ion is attracted to the now-positive environment of the axon terminal
  b. rushes out of the axon terminal, because this positively charged ion is repelled by the now-positive environment of the axon terminal.
  c. leaves the axon terminal as the action potential recedes, because that is when the calcium channels begin to open.
  d. enters the axon terminal as the action potential recedes, primarily due to diffusion.