(2013·安徽，21)From space, the earth looks blue.This is ________about seventy－one percent of its surface is covered by water.
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A
why
B
how
C
because
D
whether
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It is no longer a problem _______ the poor children in this district can go to school.
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A
that
B
whether
C
so
D
because
I wonder how many years ago ___________.
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A
did your father retire
B
has your father retired
C
your father retired
D
your father has retired
Following the girl, we went into a hall, on ____ walls hung a few pictures of some famous scientists．
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A
its
B
which
C
whose
D
those
Thanks to the application of new farming methods,output is now six times _____it was before 2000.
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A
that
B
what
C
which
D
how