单项选择题

I ______ two hours on this math problem.

A. spent
B. paid
C. cost
D. taken
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It has been ten years since she ______ a nurse.
A. is
B. has been
C. was
D. had been
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Not until the early years of the 19th century ______ know what heat is.
A. did man
B. man did
C. have man
D. man have
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