By the time human beings become fully aware of the importance of the ozone layer, it ______ seriously damaged.

Students will not be allowed into the exam room if they _____________ their student cards.

Although at that time my knowledge of German was very poor, I _____________ most of what they said.

Smoking is extremely detrimental to health, _____ many people continue to smoke anyway.

You’ll probably come _________the problems of culture shocks when you study abroad.

That style of dress _______ have been designed by Titian, because it wasn’t worn till after his death.

In Viet Nam, you shouldn't ________ at somebody's house on the first day of the New Year unless you have been invited by the house owner.

According to FAO, Vietnam is ___ second largest coffee producer in the world after Brazil.