The function $f(x) = tan x- x $ for $x \in \left(-\frac{\pi }{2}, \frac{\pi }{2} \right) $ is :

increasing function decreasing function neither increasing nor decreasing function decreasing in $\left(\frac{-\pi }{2} , o \right) $ and increasing in $\left(0, \frac{\pi }{2} \right) $

increasing function