When the concentration of molecules on both sides of a membrane is the same, the molecules will A move across the membrane to the inside of the cell
B stop moving across the membrane
C move across the membrane is both directions
D move across the membrane to the outside of the cell

B. Stop moving across the membrane.

Explanation:

Movement of molecules across the cell membrane depends on the concentration of the molecules on both sides of the membrane i.e the higher the concentration of molecules,the higher the attraction,since the molecules have the same concentration on both sides of the membrane the molecules does not move to either of the sides.