Four charges +q,+q,-`-`q and -`-`q respectively are placed at the corners A, B, C and D of a square of side a. The electric potential at the centre O of the square is
`(1)/(4piepsi_0)` `q/a`
`(1)/(4piepsi_0)` `(2q)/a`
`(1)/(4piepsi_0)``(4q)/a`
zero.
How it could be zero?