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 

   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.

1. A.
   while
2. B.
   once
3. C.
   when
4. D.
   as

   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 

   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.