The survivors of the crashed plane had no food or water and were________the extreme cold weather.[ ]

A．in the possession of  

B．in the charge of

C．at the mercy of  

D．at the risk of

骨质疏松症发生的主要机制是（）

A.钙摄入太少

B.性激素缺乏

C.衰老

D.缺乏运动

E.成骨和破骨之间的功能失衡导致骨量的丧失