11.  In 1960, life expectancy was about forty, it has been increased to sixty by 1990.

12.  And infant mortaility has been Decrease from 200 deaths per 1,000 births in 1960 to 100 death per 1,000 births in 1990.

13.  In my oponion , there are two main reasons for it .

14.  But the main reasons is: the first, people's life makes a great progress , they have many good food to eat, so that they a good health; the second, in the past, the technology didn't advance and people worked in a long time, it made them tired, the healthy was very bad.

15.  In a word the development of society is a main reason.

16.  I think there are, among others, two main reasons.

17.  I think the main reason is that prepare is not enough.

18.  The main reason is that we do not consider things wholely .

19.  There are two main reasons.

20.  This one is the main reason.