Let $a$ and $b$ be digits. Given the sets
$$\begin{aligned}
& A = \{ 5,6,7,8,9 \} \\
& B = \{ 1,4,5,7 \} \\
& C = \{ a , b \}
\end{aligned}$$
If the number of elements in the Cartesian product $(A \cup C) \times (B \cup C)$ is 28, what is the sum $a + b$?\\
A) 5\\
B) 6\\
C) 8\\
D) 9\\
E) 11