A reagent is any chemical that reacts in a predictable way _______withother chemicals.
A. when mixed
B. when is mixed
C. it mixed
D. when mixing
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As a general rule, snakes don't bite unless ________.
A. are offended
B. they offended
C. offending
D. offended
Not until ________himself seriously ill.
A. he had completed the task did he find
B. had he completed the task did he find
C. he completed the task he had found
D. did he complete the task he had found
By the time the firemen arrived, the whole building ________.
A. has burned down
B. had burned down
C. will have burned down
D. burned down
_______Charles started a job, he did it_______it was finished.
A. Since, where
B. Once, until
C. Each time, once
D. The moment, as
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