I finally found the problem. It didn't work because of the lockroot method. It allows loaded movies to lock actionscript to the _root timeline of the loaded movie. So I just needed to add :
this._lockroot = true;
to the bottom of the code of the B file to make it work. Now it will allow me to use the script of the B file into the A file. More info about this check help and look for _lockroot.
Thank you marsoul for you time. I appreciate your help.