A single fitting installed in a pipeline that either blanks off the pipe or allows a full flow passage of a liquid through the pipe is referred to as a（）.

A.blind flange

B.pivot coupling

C.quick-release coupling

D.spectacle flange

加拿大湿地面积仅次于俄罗斯，居世界第二，读图（甲乙两地海拔都小于200米），回答下列问题。

试分析甲地东北部沼泽广布的原因。