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【单选题】
旋转型步进电机可分为反应式、永磁式和感应式三种。其中 步进电机由于惯性小、反应快和速度高等特点而应用最广。A.
反应式
B.
永磁式
C.
感应式
D.
反应式和永磁式
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【简答题】Driving while talking on a hand-held mobile phone has long been considered a er. But even drivers who use hands free devices are a nuisance – because they are slowing everyone else down, according...
【单选题】She ought to stop work she has a headache because she ________ too long.
A.
has been reading
B.
had read
C.
is reading
D.
read
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