What is the key difference between a liquid and a gas? i can't decide between these two i have narrowed it down to: a. motion of particles b. average kinetic energy,

There are many differences between liquid and solid but with your remaining choices, the first one is the correct answer. motion of particles  This is because you could tell whether it is solid if the movement of the particles is slow due to its small gaps while in liquid, the movement of the particles are faster than that of the solid because it has more space between its particles.

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