Concept:Osmosis is the movement of water molecules through a selectively permeable membrane from a dilute solution (more water) to a more concentrated solution (less water).
Explanation:In the kidney tubules, useful substances and much of the water are taken back into the blood (selective reabsorption).
The water moves back into the blood by osmosis through the selectively permeable walls of the tubules.
Absorption of digested food in the small intestine is by diffusion and active transport.
The spread of petrol scent through air is diffusion of gas molecules.
Gaseous exchange (oxygen in, carbon dioxide out) is by diffusion.
Answer:Option A (Selective reabsorption)