27. ---Shall we go out for a walk? 　　　--- Sorry. This is not the right ____ to invite me. I am too tired to walk. 　　　(A) moment (B) situation 　　　(C) place (D) chance

31. It was announced that only when the fire was under control ___ to return to their homes. 　　　(A) the residents would be permitted 　　　(B) had the residents been permitted 　　　(C) would be residents be permitted 　　　(D) the residents had been permitted 　　　