Let roots of equation $x^2-ax+b$ be $\alpha,\beta$.Given $A_n=\alpha^n+\beta^n$ then,$A_{n+1}-aA_n+bA_{n-1}$ is equal to

Question

$\begin{array}{1 1}(A)\;-a&(B)\;0\\(C)\;b&(D)\;(a-b)\end{array}$