It’s polite youto say hello to your neighbours every day.

It was when we were returning home _____ I realized what a good feeling it was to have helped someone in trouble.

It is uncertain ________ side effect the medicine will bring about, although about two thousand patients have taken it.(2010·浙江)

_____ in the mountains for a week, the two students were finally saved by the police [ ]

Mom’s gift? She _____ have chosen a better one for me.—She must be very happy to know it.[ ]