Let the $f:R\rightarrow R$ be defined by $f(x)=2x+\cos x,$then

Question

$\begin{array}{1 1} (A)\;\text{has a minimum at $x=\pi$} \\ (B)\;\text{has a maximum,at x=0}\\(C)\;\text{is a decreasing function} \\ (D)\;\text{is an increasing function}\end{array}$