Which of the following matrices is not invertible ?

$\begin{bmatrix}1 & 1\\0 & 1\end{bmatrix}$

$\begin{bmatrix}2 & 3\\4 & 6\end{bmatrix}$

$\begin{bmatrix}-1 & -1\\-1 & 5\end{bmatrix}$

$\begin{bmatrix}2 & -2\\1 & 1\end{bmatrix}$