Ans: D
$\begin{array}{cl}
& \alpha^2-\alpha-\beta^2+\beta \\
= & \alpha^2 -\beta^2 -(\alpha -\beta) \\
= & (\alpha+\beta)(\alpha -\beta) -(\alpha-\beta) \\
= & (\alpha-\beta)(\alpha+\beta-1)
\end{array}$
