9. In the range of real numbers, let
$$
\begin{aligned}
x= & \left(\frac{\sqrt{(a-2)(|a|-1)}+\sqrt{(a-2)(1-|a|)}}{1+\frac{1}{1-a}}\right. \\
& \left.+\frac{5 a+1}{1-a}\right)^{1988} .
\end{aligned}
$$

Then the unit digit of $x$ is ( ).
(A) 1
(B) 2
(C) 4
(D) 6
Answer: D