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 $x_1\left(t\right)$ after time $t$ and that of the second body by $x_2\left(t\right)$ after the same time interval. Which of the following graphs correctly describes $\left(x_2-x_1\right)$ as a function of time?