He passed through a difficult period _____ after his marriage broke down. 

 People say that young Chinese students’ eyesight keeps dropping _________ because of both too much homework and poor learning conditions.

          I got over from the failure of my entrance examination. And I made up my mind to devote myself to my study again.

The secret of his success is that he does everything___________．

Food safety is ________ important, so the government spares no efforts to prevent food pollution.