Li Lei didn't meet the famous American professor ________ he was on holiday in American last year.
A. unless
B. until
C. if
D. whether
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They were obliged ________their house in order to pay the debt.
A. to sell
B. selling
C. sold
D. in selling
The manager of the company insisted that all the staff members ________ the safety rules.
A. would observe
B. observe
C. observed
D. will observe
I was away that day or I ________to see you.
A. would go
B. would have gone
C. went
D. was
He owes his success ________his mother.
A. to
B. on
C. for
D. at
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