—How did you survive the fire? —The firefighters ____my screams, as I felt myself being pulled out of the house.
A. could have heard
B. might have heard
C. must have heard
D. should have heard
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Not enough American students want to be engineers, mathematicians, _____scientists.
A. or
B. and
C. Either
D. but
I would not have chosen law as my major if I ____that it was so difficult to find a job.
A. had known
B. have know
C. Know
D. knew
_____why he was against cloning human, the scientist explained that it might lead to disaster.
Asked
B. Having asked
C. Being asked
D. To be asked
Having no one else _____ him with the work, he had to put it off till later.
A. help
B. Helped
C. helping
D. to help