If $$y = f(x)$$ and $$f(x) = \frac{(1-x)}{(1 + x)}$$, which of the following is true?
Among all options only D satisfies the given equations as follows:
$$f(y) = \frac{1-y}{1+y}$$
and for x:
$$y + xy = 1-x$$
$$x(1+y) = 1-y$$
$$x= \frac{1-y}{1+y}$$
Hence $$x=f(y)$$
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