SF6断路器广泛采用的的灭弧方式为（）。
A.压气式
B.双压式
C.真空式
D.自能式
参考答案：D
简述无领导小组讨论的概念、类型、原理和优缺点
Miss Li asked a question, but it was ________ that nobody could answer it. [ ]
A. very diffcult
B. too diffcult
C. diffcult enough
D. so diffcult
