Chọn A
\(D={\rm R}\backslash \left\{-m\right\}; y'=\frac{m-1}{\left(x+m\right)^{2} } \)
Theo yêu cầu bài toán:
\(y'<0,\forall x\in \left(2;+\infty \right)\Leftrightarrow \frac{m-1}{\left(x+m\right)^{2} } <0,\forall x\in \left(2;+\infty \right)\)
\(\Leftrightarrow \left\{\begin{array}{l} {m-1<0} \\ {-m\notin \left(2;+\infty \right)} \end{array}\right. \Leftrightarrow \left\{\begin{array}{c} {m<1} \\ {-m\le 2} \end{array}\Leftrightarrow -2\le m<1\right. .\)