7.116 Let $f(t)=\frac{t}{1-t}, t \neq 1$, if $y=f(x)$, then $x$ can be expressed as
(A) $f\left(\frac{1}{y}\right)$.
(B) $-f(y)$.
(C) $-f(-y)$.
(D) $f(-y)$.
(E) $f(y)$.
(18th American High School Mathematics Examination, 1967)
[Solution] Let $y=\frac{x}{1-x}$, we can solve for $x=\frac{y}{1+y}$. And $x=\frac{y}{1+y}=-\frac{-y}{1-(-y)}=-f(-y)$. Therefore, the answer is $(C)$.