Correct Answer - C
Some carries proteins allow transport only if two types of molecules move together. This is called cotransport. It is of two type: symport and antiport method. In symport method of cotransprot, both molecules cross the membrance in the same direction at the same time. In antiport method of cotransport, both molecules move in opposite direction. When a molecules moves across a membrane independent of other molecule, the process is called uniport.