有功电流是造成发电机感应电势和端电压幅值差的主要原因。
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Mr Black took his _____________daughter to the classroom.
A. seven-year
B. seven years
C. seven year
D. seven-year-old
There ____ pencil-box, two books and some flowers on the desk.
A. is a
B. are some
C. has a
D. have some
It’s important _______ you to learn Chinese well.
A. of
B. by
C. for
D. with
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