Which of the following statements is true?
a)during active transport, a material may be moved from an area of low concentration to an area of high concentration.
b) during passive transport, energy is used to move material against a concentration gradient.
c)during active transport, water is pumped across a membrane to equalize the concentration of salts on both sides.
d)during passive transport, osmosis allows salts to move from one side of the membrane to the other to equalize concentrations on both sides.