Question

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

Answer 1

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.