Ans: B
Since the coordinates of the mid-point of $AB$ are $(1,5)$, we have
Since the coordinates of the mid-point of $AB$ are $(1,5)$, we have
$\begin{array}{rcl}
\dfrac{a+7}{2} & = & 5 \\
a + 7 & = & 10 \\
a & = & 3
\end{array}$
