Some scientists have questioned the theory ______ CO2 is causing dangerous global warming.
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A
which
B
what
C
that
D
why
推荐题目
Children will work actively if _______. 
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A
praise
B
praised
C
praising
D
to praise
We’ll have to finish the job, _________.
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A
long it takes however
B
it takes however long
C
long however it takes
D
however long it takes