单项选择题
The United Nations Conference on Global Environment, which took place earlier this year in Vienna, was a very（） meeting.
A.productive B.communicative C.aggressive D.protective
单项选择题 We watched the plane（）behind the clouds.
单项选择题 Pipes made of plastic are now widely used in building as they are light and do not become （）in water.
单项选择题 They were under the（）that the company was doing well, but in fact it was in serious trouble.
