ctex编译出错

Command Line: tex.exe --src --interaction=errorstopmode "2.tex"
Startup Folder: C:\Users\sony\Desktop
This is TeX, Version 3.1415926 (MiKTeX 2.9)
(C:\Users\sony\Desktop\2.tex
! Undefined control sequence.
l.1 \documentclass
{article}
?
Process has been terminated ...
出现以上的报告。。求问是什么原因。。。
我的原程序只是
\documentclass{article}
\begin{document}
hello, world
\end{document}