If $x$ is real,then maximum value of $y=2(a-x)(x+\sqrt{x^2+b^2})$ is

Question

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