The three-digit number $2a3$ is added to the number $326$ to give the three-digit number $5b9$.  If $5b9$ is divisible by 9, then $a+b$ equals
$\text{(A)}\ 2\qquad\text{(B)}\ 4\qquad\text{(C)}\ 6\qquad\text{(D)}\ 8\qquad\text{(E)}\ 9$
If $5b9$ is divisible by $9$, this must mean that $5 + b + 9$ is a multiple of $9$. So, \[5 + b + 9 = 9, 18, 27, 36...\]
Because $5 + 9 = 14$ and $b$ is in between 0 and 9, 
\[5 + b + 9 = 18\]
\[b = 4\]
The question states that
\[2a3 + 326 = 549\]
so 
\[2a3 = 549 - 326\]
\[2a3 = 223\]
\[a = 2\]
\[a + b = 6\]
which is answer choice $\boxed{C}$.