The UN has warned that, natural systems like forests and wetlands disappear , mankind is

发布时间：2020-08-16 11:40:51

   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

网友回答

A. while
B. once
C. when
(答案→)D. as
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