The UN has warned that,_____ natural systems like forests and wetlands disapp
题型：不详难度：来源：
The UN has warned that,_____ natural systems like forests and wetlands disappear , mankind is losing the services they provide such as protection from extreme weather.
A．while
B．once
C．when
D．as
答案
D
解析
略
举一反三
______housing prices are going up madly, it is almost certain that the local government focuses not on improving people’s life but on trying to earn money by selling state land.
------ What happened to you yesterday evening? Why didn’t you finish your work? ------____ the lights off, we could not continue our work. As a result, we had to leave.