有下列原电池:(-)Zn|Zn2+(mol·L-1) ||Cu2+(mol·L-1)|Cu(+)，该原电池的负极反应为；正极反应为。
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Who ________ the bicycle?
A. invented
B. has invente
C. invents
D. had invented
I was unhappy because the trip was cancelled. I ___________ forward to it.
A. was looking
B. am looking
C. has been looking
D. had been looking
I'm looking for Paul. _________ him?
A. did you see
B. have you seen
C. had you seen
D. were you see
Mary __________ for me when I ______
A. was waiting, arrived
B. has been waiting, arrived
C. has waited, arrived
D. was waiting, was arrived
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