The real value of a for which the roots $x_1,x_2,x_3$ of $x^3-6x^2+ax-a=0$ satisfy $(x_1-3)^3+(x_2-3)^3+(x_3-3)^3=0$ is

Question

$\begin{array}{1 1}(A)\;3&(B)\;-3\\(C)\;9&(D)\;-9\end{array} $