To do some voluntary work for the 2nd Youth Olympic Games is ______ excellent chance for us to serve our hometown—Nanjing.

Where did you put my dictionary after using it this morning? –I put it back ______ I took it.

Mum, why not ___________ having hamburgers instead of dumplings?

We wondered __________ this afternoon.

Various ways have been provided for us to solve the problem. We can choose _____to start with.