Students work together during an experiment about Newton’s laws. The students use a setup that consists of a cart of known mass connected to one end of a string that is looped over a pulley of negligible friction, with its other end connected to a hanging mass. The cart is initially at rest on a horizontal surface and rolls without slipping when released. The inertia of the cart's wheels is negligible. Students have access to common laboratory equipment to make measurements of components of the system. By collecting the appropriate data, the students can determine the relationship between the acceleration of the cart and the net force exerted on the cart. Which of the following graphs should the students produce to show the correct relationship?