明沟排水坡度为( )。

A．0.1%～0.2%

B．0.3%～0.5%

C．3.0%～5.0%

D．4.0%～8.0％

           we can’t remove our troubles in life forever, we can leave them behind when we are on vocation.

A．When

B．While

C．Unless

D．Until