If $\begin{bmatrix} 1 & 2 & 1 \\ 2 & 3 & 1 \\ 3 & a & 1 \end{bmatrix}$ is non-singular matrix and $a \in A$, then the set A is (A) $\mathbb{R}$ (B) $\{0\}$ (C) $\{4\}$ (D) $\mathbb{R} - \{4\}$