When she came out, Mary looked a bit tired because she ______ the house.
A. cleaned
B. was cleaning
C. had cleaned
D. had been cleaning
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—Is Tom still not back?— No. By next year he _____ from home for three years.
A. will leave
B. will have been away
C. leaves
D. has been away
The boy was caught cheating in the exam and _____by his headteacher at the moment.
A. was questioned
B. has questioned
C. is being questioned
D. has been questioning
I/O区的寄存器不能直接与PLC外部的输入、输出端子传递信息。
A. 对
B. 错
继电器、电磁阀和指示器件都可以做PLC的输出元件。
A. 对
B. 错
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