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