答案：C

考查doubt的用法。doubt用于肯定句时用whether引导同位语从句；用于否定句时，用that引导。例如：There is no doubt that...“......是毫无疑问的。”；There is some doubt whether he will come.。本题是否定句，故用that，无疑他将不被允许进入这个房间.