急性阑尾炎的主要病因是（）.

A.急性腹膜炎扩散

B.机体抵抗力下降

C.全身感染

D.阑尾腔机械性梗阻

E.阑尾损伤

After walking in the Shanghai World Expo for a whole day, my legs ______ under me and

I couldn't walk any farther.[ ]

A. gave up        

B. gave out     

C. gave in         

D. gave away