问题补充：
单选题RemembertodothisintimeofH1N1---constantlythewindowstoletfreshairenterandmovearoundinside,andavoidingtheaccesstothecrowds,anecessityeveryday.A.openB.we openC.opening;D.to open
答案：
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.
