Which of the assumptions of the kinetic-molecular theory best explains the observation that a balloon collapses when exposed to liquid nitrogen (which is much colder than a cold winter day!!)?\ \ The velocity of gas molecules is proportional to their Kelvin temperature.\ In collisions with the walls of the container or with other molecules, energy is conserved.\ Gas molecules move at random with no attractive forces between them.\ Collisions with the walls of the container or with other molecules are elastic.\ The amount of space occupied by a gas is much greater than the space occupied by the actual gas molecules.