1999 Gauss 8 Problem 17

In the subtraction question, \begin{array}{r} a9 \\ \hline 18b8 \end{array} the sum of the digits a and b is

\textbf{(A)}\ 15\quad \textbf{(B)}\ 14\quad \textbf{(C)}\ 10\quad \textbf{(D)}\ 5\quad \textbf{(E)}\ 4

If there are no answer choices shown, enter a numerical answer.

Full credit to this problem is given to the CEMC, you may view all Gauss contests here.