基因治疗按其受体细胞不同分类，则有___、____两类
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He practices hard. I can hear him _____ every day.
A. to sing
B. sing
C. sings
D. to sings
He ______ to work in New York, but now he is in London.
A. used
B. uses
C. was used
D. was using
The policy of reform. and opening up has a positive ____ on Chinese economic development.
A. role
B. impact
C. impress
D. future
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