25. Let $f ( x ) = a x / ( x + 1 ) , x \neq - 1$. Then for what value of $a$ is $f [ f ( x ) ] = x$ : (A) $\sqrt { } 2$ (B) $- \sqrt { } 2$ (C) 1 (D) - 1
25. Let $f ( x ) = a x / ( x + 1 ) , x \neq - 1$. Then for what value of $a$ is $f [ f ( x ) ] = x$ :\\
(A) $\sqrt { } 2$\\
(B) $- \sqrt { } 2$\\
(C) 1\\
(D) - 1\\