Fundamentally, pressure is defined as force per unit area. What is the source of the force in a gas sample? A) The force is from gravity acting on the particles. B) The force is composed of the sum of the collisions only between gas molecules. C) The force is composed of the sum of the collisions only between the gas molecules and the container. D) The force is composed of the sum of all collisions--between the gas molecules, each other and the container. E) The force was imparted to the container in its initial filling, a transfer of momentum.