# A discontinuous function $y=f(x)$ satisfying $x^2+y^2=4$ is given by $f(x)$=_________
$\begin{array}{1 1}(a)\;\left\{\begin{array}{1 1}\sqrt{4-x^2},&-2\leq x\leq 0\\-\sqrt{4-x^2}&0\leq x\leq 2\end{array}\right.&(b)\;\left\{\begin{array}{1 1}\sqrt{x^2-4},&-2\leq x\leq 0\\-\sqrt{4-x^2}&0\leq x\leq 2\end{array}\right.\\(c)\;\left\{\begin{array}{1 1}\sqrt{4-x^2},&2\leq x\leq 0\\-\sqrt{4-x^2}&0\leq x\leq 2\end{array}\right.&(d)\;\left\{\begin{array}{1 1}\sqrt{4-x^2},&-4\leq x\leq 0\\-\sqrt{4-x^2}&0\leq x\leq 2\end{array}\right.\end{array}$