Why is this simple exam document couldn't be compiled?

I am learning how to type an exam under the exam documentclass, below is how I typed it, but it couldn't be compiled and I have no clue why.

\documentclass[11pt,addpoints,a4paper]{exam}

\begin{questions}
\question Why is there air?
\question What if there were no air?
\end{questions}


I think there shouldn't be any mistake and I have checked that I have the exam.cls.
Many thanks for any help, I will really appreciate it!

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As with all LaTeX documents, you need to include \begin{document}...\end{document} as follows:

% arara: pdflatex