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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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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