Two identical discs of same radius $R$ are rotating about their axes in opposite directions with the same constant angular speed $\omega$. The discs are in the same horizontal plane. At time $t = 0$, the points $P \& Q$ are facing each other as shown in the figure. The relative speed between the two points $P \& Q$ is $v_r$ . In one time period ($T$) of rotation of the discs, $v_r$ as a function of time is best represented by