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

作为在校园意外伤害事故中特殊主体学校而言，其减责和免责条件主要包括不可抗力、意外事件和第三人的过错。