6. Simplify $\frac{\left(x^{2}+1\right)\left(x^{2}-1\right)}{x^{3}+x^{2}+x+1}$, we get $(\quad)$.
(A) $x-1$
(B) $x+1$
(C) $1-x$
(D) $-1-x$
(E) *
6. (A) 

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