The coordinate of the circum centre of the triangle formed by the points $0(0,0), A(a,0)$ and $B(o,b)$ are
$\begin{array}{1 1}(A) \;(a,b) \\(B)\;\bigg(\large\frac{a}{2},\frac{b}{2}\bigg) \\(C)\;\bigg( \large\frac{b}{2}, \frac{a}{2} \bigg) \\(D)\;(b,a) \end{array}$