Two identical discs of same radius R are rotating about their axes...

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 $P$ and $Q$ are facing each other as shown in the figure. The relative speed between the two points $P$ and $Q$ is $v_{r}$. In one time period $(T)$ of rotation of the discs, $v_{r}$ as a function of time is best represented by