Given the matrices $A=\left[\begin{array}{cc}-1 & 2 \\ 3 & 4\end{array}\right]$ and $B=\left[\begin{array}{cc}0 & -4 \\ 4 & 0\end{array}\right]$
A. It is Select an answer $\checkmark$ to add these matrices.
B. If addition is possible what is $A+B$

Step 1 ：Given two matrices A and B, where A = $\left[\begin{array}{cc}-1 & 2 \\ 3 & 4\end{array}\right]$ and B = $\left[\begin{array}{cc}0 & -4 \\ 4 & 0\end{array}\right]$

Step 2 ：To add two matrices, they must have the same dimensions. In this case, both A and B are 2x2 matrices, so addition is possible.

Step 3 ：The addition of two matrices is done element by element. That is, the element in the i-th row and j-th column of the resulting matrix is the sum of the elements in the i-th row and j-th column of the two original matrices.

Step 4 ：So, to find A + B, we add the corresponding elements of matrices A and B: $A + B = \left[\begin{array}{cc}-1 & 2 \\ 3 & 4\end{array}\right] + \left[\begin{array}{cc}0 & -4 \\ 4 & 0\end{array}\right]$

Step 5 ：Performing the addition, we get: $= \left[\begin{array}{cc}(-1+0) & (2+(-4)) \\ (3+4) & (4+0)\end{array}\right]$

Step 6 ：Simplifying the above expression, we get: $= \left[\begin{array}{cc}-1 & -2 \\ 7 & 4\end{array}\right]$

Step 7 ：So, $A + B = \left[\begin{array}{cc}-1 & -2 \\ 7 & 4\end{array}\right]$. This is the simplest form and meets the requirements of the problem.

Step 8 ：Final Answer: $\boxed{\left[\begin{array}{cc}-1 & -2 \\ 7 & 4\end{array}\right]}$