Which one of the following matrices is singular?

1. $\begin{bmatrix} 2 & 5 \\ 1 & 3 \end{bmatrix}$
2. $\begin{bmatrix} 3 & 2 \\ 2 & 3 \end{bmatrix}$
3. $\begin{bmatrix} 2 & 4\\ 3 & 6 \end{bmatrix}$
4. $\begin{bmatrix} 4 & 3\\ 6 & 2 \end{bmatrix}$

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