Solve this following

Mark $(\sqrt{ })$ against the correct answer in the following:

$f(\mathrm{x})=\sin \mathrm{x}-\mathrm{kx}$ is decreasing for all $\mathrm{x} \notin \mathrm{R}$, when

A. $k<1$

B. $k \leq 1$

C. $k>1$

D. $\mathrm{k} \leq 1$