若实数$x$，$y$满足不等式组$\left\{\begin{array}{l}\begin{array}{c}x+3y-3\ge 0\\ 2x-y-3\le 0\\ x-my+1\ge 0\end{array}\\ \end{array}\right.$且$x+y$的最大值为9，则实数$m=$（）

A．

B．

C．

1

D．

2