The distance between the points $(a,b)$ and $(-b,a)$ is _______

Question

$\begin{array}{1 1}(A) \; \sqrt {a^2+b^2}\\(B)\;\sqrt {a^2-b^2}\\(C)\;\sqrt {2(a^2+b^2)} \\(D)\;\sqrt {2 (a^2-b^2)} \end{array}$