A family of five is really a heavy burden, so we all have to work  hard to________.

 As is known, mobile phones make it possible for people to ______ each other conveniently.

. The police refused to _____ the clues they were working on.

I soon found that the work I was doing had already been done by someone else－    , I was wasting my time.

Just as the experts _______ it, this kind of material is not natural, but man-made.