For two non-zero real numbers $a , b$, two square matrices $A , B$ satisfy $A B = \left( \begin{array} { l l } a & 0 \\ 0 & b \end{array} \right)$. Which of the following in <Remarks> are correct? [4 points]
\textbf{<Remarks>}\\
ㄱ. If $a = b$, then the inverse matrix $A ^ { - 1 }$ of $A$ exists.\\
ㄴ. If $a = b$, then $A B = B A$.\\
ㄷ. If $a \neq b$ and $A = \left( \begin{array} { l l } 1 & 0 \\ 1 & 1 \end{array} \right)$, then $A B = B A$.\\
(1) ᄀ\\
(2) ᄃ\\
(3) ᄀ, ᄂ\\
(4) ㄴ, ㄷ\\
(5) ᄀ, ᄂ, ᄃ