Which of the following functions is an involution (its own inverse) on R∖{0}\mathbb{R} \setminus \{0\}R∖{0}?
f(x)=1/xf(x) = 1/xf(x)=1/x
f(x)=−xf(x) = -xf(x)=−x
f(x)=1−xf(x) = 1-xf(x)=1−x
f(x)=2xf(x) = 2xf(x)=2x