A thin semi-circular ring of radius r has a positive charge q distributed uniformly over it. The net field `vecE` at the centre O is
A. `q/(4pi^(2) epsi_(0) r^(2)) hat(j)`
B. `-q/(4pi^(2) epsi_(0) r^(2)) hat(j)`
C. `-q/(2pi^(2) epsi_(0) r^(2)) hat(j)`
D. `q/(2pi^(2) epsi_(0) r^(2)) hat(j)`
