Diffusion occurs because
a molecules are attracted to one another.
b. molecules constantly move and collide with each other pushing each other along the concentration gradient.
c. cellular energy forces molecules to collide with each other
d. cellular energy pumps molecules across the cell membrane.

The correct answer is B molecules constantly move and collide with each other pushing each other along the concentration gradient.

Explanation:

Diffusion is a process by which the molecules in a system moves from high concentration region to the region of low concentration until the concentration of that molecule on both sides become same or equal.

   No external energy is required to carry out diffusion as the molecules those are transported through diffusion moves along the concentration gradient.

  Diffusion helps in the transport of small molecules ans gases such as oxygen and carbon dioxide across the plasma membrane.

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explanation: