If the roots of the equation $(a^2+b^2)x^2-2b(a+c)x+(b^2+c^2)=0$ are equal,then

Question

$\begin{array}{1 1}(A) \; 2b=a+c\\(B)\;b^2=ac \\(C)\;b=\large\frac{2ac}{a+c} \\(D)\;b=ac \end{array}$