Ans: D
$\left\{ \begin{array}{ll}
m+2 = n-1 & \ldots \unicode{x2460} \\
m+2 = 3m+n-46 & \ldots \unicode{x2461}
\end{array} \right.$
$\left\{ \begin{array}{ll}
m+2 = n-1 & \ldots \unicode{x2460} \\
m+2 = 3m+n-46 & \ldots \unicode{x2461}
\end{array} \right.$
From $\unicode{x2460}$, we have
$\begin{array}{rcl}
m + 2 & = & n – 1 \\
m & = & n – 3 \ldots \unicode{x2462}
\end{array}$
Sub. $\unicode{x2462}$ into $\unicode{x2461}$, we have
$\begin{array}{rcl}
(n-3) + 2 & = & 3(n-3) + n -46 \\
n-1 & = & 3n – 9 + n -46 \\
-3n & = & -54 \\
n & = & 18
\end{array}$
