AMC 12 2005a Problem 9

There are two values of $a$ for which the equation $4x^2 + ax + 8x + 9 = 0$ has only one solution for $x$. What is the sum of those values of $a$?

$\textbf{(A) } -16 \textbf{(B) } -8 \textbf{(C) } 0 \textbf{(D) } 8 \textbf{(E) } 20$