When would a cell be expected to use an active transport mechanism to move a substance from the outside environment into the cytoplasm? A: when a concentration gradient exists and more of the substance is on the inside of the cell

B: when a concentration gradient exists, and the same amount of substance is found inside and outside the cell

C: when no concentration gradient for the substance exists across the cell membrane

D: when a concentration gradient exists and more of the substance is on the outside of the cell