Which of these functions is an involution (its own inverse) on the domain R∖{0}\mathbb{R} \setminus \{0\}R∖{0}?
f(x)=1xf(x) = \frac{1}{x}f(x)=x1
f(x)=−xf(x) = -xf(x)=−x
f(x)=11−xf(x) = \frac{1}{1-x}f(x)=1−x1
f(x)=1−xf(x) = 1-xf(x)=1−x