A weather tower receives no radar signal in an area. Which type of weather are you most likely to see in the area?
A. Rain
B. A tornado
C. A thunderstorm
D. Sunshine

Diffusion happens against a concentration gradient. Diffusion of a particles occurs when it moves from a region of higher concentration to region on lower concentration. Diffusion occurs as long as there is a difference in concentration. Diffusion stops as soon as there is no concentration gradient.  

There is no diffusion of  H⁺ ions when the channel opens as the concentration of H⁺ ions are the same on both sides of the membrane.