Find the equation of the circle with centre (–a, –b) and radius $ \sqrt{a^2-b^2}$

Question

$\begin {array} {1 1} (A)\;x^2+y^2-2ax-2ay+2b^2 & \quad (B)\;x^2+y^2-2ax-2ay+2b^2=0 \\ (C)\;x^2+y^2+2ax+2ay+2b^2=0 & \quad (D)\;x^2+y^2-2ax-2ay-2b^2=0 \end {array}$