Once________，it can never be forgotten．

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The flu is believed________ by viruses that like to repro-duce in the cells inside the human nose and throat．

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The disc，digitally________ in the studio．sounded fantastic at the party that night．

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Having been attacked by terrorists，________．

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A．doctors came to their rescue

B．the tall building collapsed

C．an emergency measure was taken

D．warnings were given to tourists

“we can't go out in this weather．”said Bob，________out of the window．

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He got well-prepared for the job interview，for he couldn't risk________ the good opportunity．

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You should listen carefully and be patient when________．

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This was one of the buildings ________ in 2004．Next year we'll have another building．

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Peter has several brothers，of whom Jack is________．

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We sent a letter of thanks to ________ had helped us．

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