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  • 更新 2022-09-03
  • 科目 英语
  • 题型 单选题
  • 难度 中等
  • 浏览 498

   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
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TheUNhaswarnedthat,naturalsyst