The floor does not look so bad when ________ （sweep） clean.
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排球场区由前场区和后场区之分它们是以什么来区分的（）
A. 场地的中间
B. 场地的中线
C. 限止线
We were busy ________ （get） things ready for the trial production when he phoned us.
Some molecules are large enough ________ （see） under the electronic microscope.
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