When traveling alone in the mountains, you ’d better take a _______ with you in case you get lost.

This river is so big that it is impossible to build a _______under it without modern technology.

ProfessorSmithisalsothe_______oftheinternationalprogramoffice.Ifyouhaveanyproblemwhenyoustudyhere,youmaygotohimforhelp.

He always has a lot of _______ ideas in his mind , and sometimes we do not even know what he is thinding about.

It is not ---_______ for me to return all the books to the library now because I still need some of them for my research.