答案：B
$\left\{\begin{array}{ll}
m+2n+6=7 & \ldots \unicode{x2460} \\
2m-n=7 & \ldots \unicode{x2461}
\end{array}\right.$
$\left\{\begin{array}{ll}
m+2n+6=7 & \ldots \unicode{x2460} \\
2m-n=7 & \ldots \unicode{x2461}
\end{array}\right.$
從 $\unicode{x2460}$，可得
$\begin{array}{rcl}
m + 2n + 6 & = & 7 \\
m & = & 1-2n~\ldots \unicode{x2462}
\end{array}$
把 $\unicode{x2462}$ 代入 $\unicode{x2461}$，可得
$\begin{array}{rcl}
2(1-2n)-n & = & 7 \\
2- 5n & = & 7 \\
n & = & -1
\end{array}$
