21.① many times, so he could understand it.

② many times, he couldn’t understand it.

A. Having been told B. Having told

C. He had been told D. Though he had been told 22.①The writer and scientist  present at the meeting.

②The writer and the scientist present at the meeting.

A. were B. was

C. has D. had

23. ①Three years later he turned doctor.

②Three years later he became doctor.

A. an B. a

C. 不填 D. the

24.①My brother often plays football after school.

②My brother often plays piano after school.

A. 不填 B. a

C. the D. an

25.①She is good a teacher that we all like her .

②She is good teacher that we all like her.

A. so 　B. such C. such a 　D. quite a

26. ①She often goes to school bike.

②She often goes to school foot.

A. in B. at C. on D.by

27.①More than 70 percent of the population of these country peasants.

②More than 70 percent of the population of this country  farming-population.

A. has B. have C. is D. are

28. ①He as well as his brother football now.

②He and his brother football now.

A. is playing B. is played

C. are playing D. are played

29. ①All of the apples rotten.

②All of the apple rotten.

A. are B. is

C. have been D. has been

30.①Do you know John is getting on well with his studies?

②Do you know John is getting on with his studies?

A. how B. which C. what D. that