Answer:
The answer is A. The force driving simple diffusion is the concentration gradient, while the energy source for active transport is ATP.
Explanation:
For simple diffusion, the movements of solutes and solvents depend on the electrochemical gradient. Usually, there are no difficulties for this kind of diffusion as they can pass freely through the semi-permeable membrane. This includes water, oxygen, carbon dioxide, etc. To demonstrate, we put a glass of water with sugar particles in it and a semi-permeable barrier in between, with the left side having MORE sugar. Equilibrium must always be attained in this kind of reaction. So the water (solvent) will pass through the membrane, right to left, from a higher concentration to lower concentration of water, to attain equilibrium. This doesn't require ATP or energy to occur.
For active transport, ATP will always be utilized to occur. An example would be the Na+/K+ pump. In order to maintain equilibrium, these ions are supposed to travel in and out of a cell. But moving from a low concentration to a high concentration area is difficult without a pump and ions are too big to pass through the semi-permeable membrane. So that's why they utilize this kind of transport.
