10 question is a doubt

10.  In the adjacent circuit, AB is a potentiometer wire of length 40 cm and resistance per unit length 50 $\Omega /m.$ As shown in the figure, the free end of an ideal voltmeter is touching the potentiometer wire. What should be the speed of the jockey as a function of time so that reading in the voltmeter varies with time as $\left(2 \sin \mathrm{\pi t}\right) ?$

$$\begin{gathered} \left(A\right) \left(10 \mathrm{\pi } \sin \mathrm{\pi t}\right) cm/s \left(B\right) \left(10 \mathrm{\pi } \cos \mathrm{\pi t}\right) cm/s \\ \left(C\right) \left(20 \mathrm{\pi } \sin \mathrm{\pi t}\right) cm/s \left(D\right) \left(20 \mathrm{\pi } \cos \mathrm{\pi t}\right) cm/s \end{gathered}$$