平板变形的根本原因是（）。
A.重力
B.应力
C.压力
D.压强
参考答案：B
— You may find the key to the maths problem on page 107.
— Ah, it's so simple. I wonder why I _____ of that. [ ]
A. hadn't thought
B. haven't thought
C. didn't think
D. wasn't thinking
