20. Once environmental damage(), it takes many years for the system to recover.

13、We should not look down ________ the poor people.

12、It is because he is kind and modest ________ he wins the respect of all presents.

11、He helped me ______ my homework.

10、These flowers ________ a fragrant perfume.

9、Mathematics ________ study or science of numbers.

8、We offered him our congratulations _______ his passing the college entrance exams.

7、We＇ve ________ paper and ink. Ask Mr. Howard to lend us some.

6、Every means _______ been tried to solve the difficult problem.

5、Each new way of looking at a situation ________ our understanding and makes it easier to discover new possibilities.

4、Modern machinery _______ been installed in this newly-built factory.