Before I installed some packages and add some configurations in .emacs, the .tex file in Emacs is in LaTex mode. After I changes .emacs, the .tex file is opened in Fundamental mode. All the functionalities of Latex mode works well, like compile and view. If I enable the LaTex-Math mode, the Fundamental mode will become LaTex/MP mode, and when I disable the LaTex-Math mode, it becomes LaTex/P mode.
How can I open a .tex file with LaTex mode, rather than Fundamental mode?