\textbf{109.} The distance of point $A$ from the line $x + y = a$ is $a$. The two points $B(-3, 2)$ and $C(-1, 4)$ are on this line, and $5$ is the distance. What is the value of $a$?
(1) $2$ \qquad (2) $\dfrac{1}{2}$ \qquad (3) $-\dfrac{1}{2}$ \qquad (4) $-2$