Which of the following statements best explains the processes of passive and active transport?

a. Passive transport is the net movement of substances down a concentration gradient that requires metabolic energy. Active transport is the movement of substances up a concentration gradient that does not require energy.
b. Passive transport is the net movement of substances down a concentration gradient that does not require metabolic energy. Active transport is the movement of substances against a concentration gradient that requires energy.
c. Passive transport is the net movement of substances against a concentration gradient that requires metabolic energy. Active transport is the movement of substances down a concentration gradient that does not require metabolic energy.
d. Passive transport is the net movement of substances against a concentration gradient that does not require metabolic energy. Active transport is the movement of substances down a concentration gradient that requires energy.