Today there are more airplanes ______ more people than ever before in the skies.
World Wildlife Fund ______ very much the efforts that China has made to protect endangered species from being further endangered.
It’s wise to forget things which aren't worth ______.
With the fast development of computer and Internet technologies, traditional printed books ______ by the growing popularity of online reading of e-books.
In many people’s opinion, that company, though relatively small, is pleasant ______.
The computer system _____suddenly while he was surfing the Internet.