A thin semi-circular ring of radius r has a positive charge q distributed uniformly overit. The net electric field `vecE` at the centre O is
Options:
(a) `q/(4pi^2epsilon_0r^2)hatj`
(b) `-q/(4pi^2epsilon_0r^2)hatj`
(c) `-q/(2pi^2epsilon_0r^2)hatj`
(d) `q/(2pi^2epsilon_0r^2)hatj`