well-known to us all that knowledgeis __ treasure, but practice is ___ key to it.

Thefact has worried many scientists ______ the earth is becoming warmer and warmerthese years.

Why do you look so worried—My computer broke down last week and my essay has been left________since

Why not go to Lao She Teahouse tonight

You should not mention it in the beginning, or it will ________ the surprising ending.