A body is at rest at x=0. At t=0, it starts moving in the positive X -direction with a constant acceleration. At the same instant, another body passes through x=0 moving in the positive X -direction with a constant speed. The position of the first body is given by x1(t) after time t and that of the second body by x2(t) after the same time interval. Which of the following graphs correctly describes x2-x1 as a function of time?