6. If $F\left(\frac{1-x}{1+x}\right)=x$, then which of the following equations is correct?
(A) $F(-2-x)=-2-F(x)$
(B) $F(-x)=F\left(\frac{1-x}{1+x}\right)$
(C) $F\left(x^{-1}\right)=F(x)$
(D) $F(F(x))=-x$