Final answer:
The correct answer is A. They move in random directions. According to the kinetic-molecular theory of gases, gas particles are in constant rapid motion in random directions until they collide with other molecules or the walls of the container.
Step-by-step explanation:
The correct answer is A. They move in random directions. According to the kinetic-molecular theory of gases, gas particles are in constant rapid motion in random directions until they collide with other molecules or the walls of the container. This random motion allows for gases to spread out and fill the available space. The collisions between gas particles and with the container walls are elastic collisions, meaning there is no net loss of energy from the collisions.