Four-point charges $-Q$, $-q$, $2q$ and $2Q$ are placed, one at each corner of the square. The relation between $Q$ and $q$ for which the potential at the centre of the square is zero is:
- A$Q=-q$
- B$Q=-\frac{1}{q}$
- C$Q=q$
- D$Q=\frac{1}{q}$
✓ Correct answer: A
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