问题
选择题
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
答案
答案：C
“---”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.