which of the following must be true for diffusion to occur?

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The answer is C. This is because diffusion occurs when particles travel from an area of high concentration to areas of low concentration. For example, in a cell, the cell membrane can let several material in and out. Some materials are small enough that they don't need energy to get into or out of the cell. This is called passive transport. Active transport is another type of transport in which the transportation of particles use energy (usually ATP A.K.A. Adenosine Thriphospate.) Because passive transport requires two different concentration amounts, diffusion must have two different concentration levels for it to occur, leaving us with the answer choice of C. ~MathGeek 

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