QUESTION $5$

How many solutions for $x$ are there for the equations $Ax=b$ given the following A and $b?$

$A=\left[\begin{array}{ccc}1& 2& 1\\ 0& 1& 0\\ 3& 2& 3\end{array}\right],b=\left[\begin{array}{c}12\\ 4\\ 20\end{array}\right]$

A$\mathrm{.}0$ B$.$Infinitely many C$.1$