A thin semi-circular ring of radius r has a positive charge q distributed uniformly over it. The net field E at the centre O is

Question

( A ) $\large\frac{q}{4 \pi^2 \in_0 r^2}$$\hat j$
( B ) $\large\frac{q}{2 \pi^2 \in_0 r^2}$$\hat j$
( C ) $\large\frac{-q}{4 \pi^2 \in_0 r^2}$$\hat j$
( D ) $\large\frac{-q}{2 \pi^2 \in_0 r^2}$$\hat j$