Let $R^* = R - \left\{ (2k - 1) \frac{\pi}{2} \mid k \in I \right\}$. The function $f: R^* \rightarrow R$ is defined as $f(x) = \tan x - x$, then $f(x)$ is

• A
  an increasing function
• B
  a decreasing function
• C
  minimum at $x = 0$
• D
  periodic function