Please help Which condition will calls particles to diffuse faster across a membrane

A: an increase in size of the particles
B: The same amount of particles on each side
C: Cooling of the temperature
D: A steep concentration gradient

the answer would be a

hurricanes form when warm, moist air from the ocean surface quickly rises into the atmosphere and forms storm clouds and rain. when the clouds are formed, heat is released which warms the cool air in the upper regions of the atmosphere so that the cycle can continue. which two phases of the water cycle are repeated in this process?