In the diagram, the two points $P$ and $Q$ are positioned on a circle of radius $r$ so that they are $90^\circ$ apart. A positive point charge at the centre of the circle produces an electric field of magnitude $E$ at both $P$ and $Q$. Which expression gives the work done when a unit positive charge is moved from $P$ to $Q$?
- A$0$
- B$E \times r$
- C$E \times \left(\frac{\pi r}{2}\right)$
- D$E \times (\pi r)$