Which of these statements about ion movement through the plasma membrane is true?

Which of these statements about ion movement through the plasma membrane is true?
a. Movement of Na+ out of the cell requires energy (ATP).

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b. When Ca2+ bind to proteins in ion channels, the diffusion of Na+ into the cell is inhibited.
c. Specific ion channels regulate the diffusion of Na+ through the plasma membrane.
d. All of the above are true.