Last term our maths teacher set so difficult an examination problem _____ none of us worked it out. 

A．that

B．as

C．so that

D．which

低压架空线路常用绝缘子有（）.

A.针式绝缘子

B.蝶式绝缘子

C.拉线式绝缘子

D.悬式绝缘子