Which of the following best describes the process of diffusion? a. diffusion is the movement of molecules from an area of higher concentration to one of lower concentration by random molecular motion. b. diffusion is the movement of molecules from an area of lower concentration to one of higher concentration by random molecular motion. c. diffusion is the movement of molecules from an area of lower concentration to one of higher concentration caused by an input of energy to the system. d. diffusion is the movement of molecules from an area of higher concentration to one of lower concentration caused by an input of energy to the system.