Tom ______ in the library every night over the last three months.
A. works
B. worked
C. has been working
D. had been working
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Did you predict that many students ______ up for the dance competition?
A. would sign
B. signed
C. have signed
D. had signed
By the time you arrive this evening, I ____ for two hours.
A. will study
B. will have been studied
C. had studied
D. will have been studying
请问二极管最典型的特性是什么?
A. 光生伏特效应
B. 单向导电性
C. 双向导电性
D. 折射效应
直流电动机分为哪两种类型电动机?
A. 永磁式
B. 电磁式
C. 永动式
D. 异步式