The diagram illustrates two points $P$ and $Q$ on a circle of radius $r$, separated by an angle of $90^\circ$. A positive point charge placed at the centre of the circle produces an electric field of magnitude $E$ at both $P$ and $Q$. Which expression represents the work done in transferring a unit positive charge from $P$ to $Q$?
- A$0$
- B$E \times r$
- C$E \times \left(\frac{\pi r}{2}\right)$
- D$E \times (\pi r)$