Burglar alarms ________ at any time of the day ornight serve only to annoy.

31．Every solid object______reflect a sound，varying according to the size and nature of the object．

Don’t forget_________ your homework toschool tomorrow.—____________.

How does he usually spend his weekends—He spends as much time as he can _______ computer games .

National parks in our country________these rare tigers________making special laws.