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

（）是指建筑中心线或其墙.柱等，在不同高度的点对其相应底部点的偏移现象。

A.沉降

B.位移

C.倾斜

D.扰度