For two matrices $A = \left( \begin{array} { l l } 2 & 0 \\ 1 & 0 \end{array} \right) , B = \left( \begin{array} { r r } a & 0 \\ 2 & - 3 \end{array} \right)$, when the sum of all components of matrix $A + B$ is 6, what is the value of $a$? [2 points]\\
(1) 1\\
(2) 2\\
(3) 3\\
(4) 4\\
(5) 5