At the rim of an insulating disc of radius $r$, four point charges, each with charge $Q$, are situated. The disc turns at a rate of $n$ revolutions per unit time. What is the equivalent electric current at the rim of the disc?
- A$4Qn$
- B$\frac{4Q}{n}$
- C$8\pi r Q n$
- D$\frac{2Qn}{\pi r}$