My father began to work ____ a bus driver when he was twenty years old.
Only when they were put in prison and lost freedom(自由) _________ its importance.
The Internet is a very useful invention for us, ______ we can do lots of things at home.
Science and technology are developing ______ quickly that some new electronic products are out-of-date within several months.
— Hi, Henry, I’m going to China for my holiday. — China? It is a country which is well worth .
You should never lose ______ heart even if you are in ______ great trouble.