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 $\left( {{x_1} - {x_2}} \right)$ as a function of time $'t'$ ?