7. For $x \in R$, define the sign function $\operatorname { sgn } x = \left\{ \begin{array} { c } 1 , x > 0 \\ 0 , x = 0 \\ - 1 , x < 0 \end{array} \right.$, then\\
A. $ \{ x | = x | \operatorname { sgn } x \mid \}$
B. $ \{ x | = \operatorname { sgn } | x \mid \}$
C. $ \{ x | = x | \operatorname { sgn } x \}$
D. $ \{ x \mid = x \operatorname { sgn } x \}$\\