骨质疏松症是以（），（）为特征。
参考答案：骨量减少；骨微观结构退化
No sooner had I come to the office _____ the boss told me that I was fired.
A．when
B．after
C．than
D．before
什么是塑性材料、脆性材料，各有什么特点？
