1. The problem statement, all variables and given/known data In an experiment to demonstrate Brownian motion in a gas, a brightly illuminated cell containing smoke is viewed under a microscope. The observer sees a large number of bright specks undergoing random motion. Which one of the following statements about this experiment is correct? A) Light is being scattered by gas molecules. B) The larger the smoke particle, the greater is the speed of the bright specks. C) The lower the smoke pressure of the gas, the more frequent are the direction changes of the bright specks D) The higher the temperature of the gas, the faster is the motion of the bright specks. 2. Relevant equations None 3. The attempt at a solution I chose D, the answer is D. D is obviously correct. I know B is incorrect. But I am not very sure about A and C. "A) Light is scattered by gas molecules" Isn't that so? Light is reflected in many directions by many gas molecules so it is scattered. "C) The lower the smoke pressure, the more frequent are the direction changes of the bright specks" If smoke pressure is lower, it means there are fewer smoke particles. But how does that affect the rate of direction changes?