$2 \cdot 94$ for the equation $x^{2}-2|x|+2=m$. If the equation has exactly 3 real roots, then the value of $m$ is
(A) 1 .
(B) $\sqrt{3}$.
(C) 2 .
(D) 2.5 .
(China Beijing Junior High School Mathematics Competition, 1991)
[Solution] Rewrite the equation as $|x|^{2}-2|x|+2-m=0$, then $|x|=1 \pm \sqrt{m-1}$,

i.e., $x=1 \pm \sqrt{m-1}$ or $x=-1 \mp \sqrt{m-1}$. When $m=2$, $x=2,0,-2$, there are exactly 3 roots. Therefore, the answer is $(C)$.