What is the domain of $f(x)=\large\frac{[x]+1}{[x]-1}$ where $[x]$ is greatest integer function

Question

$\begin{array}{1 1} (1,20) \\ [1,2] \\ (-\infty,1)\cup [2,\infty) \\ (-\infty,1]\cup [2,\infty) \end{array}$