For the matrix $A = \left[ {\begin{array}{*{20}{c}}1&1&0\\1&2&1\\2&1&0\end{array}} \right]$, which of the following is correct

If the matrix $AB = O$, then

If $A\, = \,\left[ {\begin{array}{*{20}{c}}
0&{ – 1}\\
1&0
\end{array}} \right],$ then which one of the following statements is not correct?

$A = \left[ {\begin{array}{*{20}{c}}4&6&{ – 1}\\3&0&2\\1&{ – 2}&5\end{array}} \right]$,$B = \left[ {\begin{array}{*{20}{c}}2&4\\0&1\\{ – 1}&2\end{array}} \right],\,\,C = \left[ {\begin{array}{*{20}{c}}3\\1\\2\end{array}} \right]$, then the expression which is not defined is

Let $p$ , $q$ , $r$ are three real numbers satisfying $\left[ {p\,\,q\,\,r} \right]\left[ {\begin{array}{*{20}{c}}
  2&p&q \\ 
  { – 3}&q&{ – p + r} \\ 
  {12}&r&{ – q + 3r} 
\end{array}} \right] = \left[ {5\,\,\,b\,\,c} \right]$ , then minimum value of $(b + c)$ is