Remember to do this in time of H1N1 --- constantly the windows to let fresh air enter and move around inside, and avoiding the access to the crowds, a necessity everyday.A.openB.we openC.opening;D.to open
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C解析“---”后是this的同位语，即名词性短语，而且根据and后面的avoiding the access to the crowds，（and前后结构对称），所以选C.