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. open
B. we open
C. opening;
D. to open
网友回答
A. open
B. we open
(答案→)C. opening;
D. to open
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.