21.  healthy diet is very important for people.

22.  We all were busy doing the maths excises day and night.

23.  When I saw t, I said it was easy, and I had done it.

24.  At last, I found the rror is in the printing.

25.  I had believes that it was a very hard question which few people could work out.

26.  " I thought I was lucky, because , when I work it yesterday .

27.  My eyes were so tired that I believed the question had the latter "I" by mistake.

28.  The composition was about how to protect the environment and it must be completed before next Wednesday.

29.  They would be finished in two weeks.

30.  That's great.