21. _______ is the center of our planetary system was a difficult concept to grasp in the Middle Age.
　　A. It is the sun and not the earth
　　B. Being the sun and not the earth
　　C. The sun and not the earth
　　D. That the sun and not the earth
　　22. _______ it or not, his discovery has created a stir in scientific circles.
　　A. Believe
　　B. To believe
　　C. Believing
　　D. Believed
　　23. _______ kinds of matter in the world.
　　A. There is a few million
　　B. That there are millions
　　C. There are a few million
　　D. It is millions
　　24. _______ man can now create radioactive elements, there is nothing he can do to reduce their radioactivity.
　　A. As
　　B. Whether
　　C. While
　　D. Now that
　　25. ______ me, I think an English-Chinese dictionary is much more helpful to our study than an All-English dictionary.
　　A. As far as
　　B. As far
　　C. As for
　　D. As regard
　　26. _______ my return, I learned that Professor Smith had been at the Museum and would not be back for several hours.
　　A. At
　　B. With
　　C. On
　　D. During
　　27. _______ of the burden of ice, the balloon climbed up and drifted to the South.
　　A. To be free
　　B. To free
　　C. Freeing
　　D. Freed
　　28. _______ one time, Manchester was the home of the most productive cotton mills in the world.
　　A. On
　　B. By
　　C. At
　　D. Of
　　29. _______ quite recently, most mothers in Britain did not take paid work outside the home.
　　A. Until
　　B. Before
　　C. From
　　D. Since
　　30. _______ she first heard of the man referred to as a specialist.
　　A. That was from Stephen
　　B. It was Stephen whom
　　C. It was from Stephen that
　　D. It was Stephen that

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