The kinetic-molecular theory of gases assumes that A. The particles are much smaller than the distance between particles. Most of the volume of a gas is therefore empty space. B. Collisions between gas particles or collisions with the walls of the container are perfictly elastic. None of the energy of a gas particle is lost when it collides with another particle or with the walls of the container. C. Different particles have different speed. D. The pressure of a gas results from collisions between the gas particles and the walls of the container. So, option (A) and (D) are only correct.