Find the length of common chord of the two circles $(x-p)^2+(y-q)^2=c^2$ and $(x-q)^2+(y-p)^2=c^2$.

Question

$\begin{array}{1 1}(a)\;\sqrt{4c^2-2(p-q)^2}\\(b)\;\sqrt{4c^2+2(p-q)^2}\\(c)\;\sqrt{4c^2-2(p+q)^2}\\(d)\;\sqrt{6c^2-(p-q)^2}\end{array}$