Here’s a _____ in our office that when it’s somebody’s birthday, they bring in a cake for us all to share.

I heard a soft _______ at the door. When I looked up, I found it was Mary. 

Thanks is a common _____ used in English. You can never use it too much.

The government has raised the tax on urban building (房产税) with the ______ of controlling the price of buildings.

We should bear in mind that social development and balance of nature should go  in         .