Though young, he succeeded in taking        the machine skillfully and then put it together without any difficulty.

A．out

B．down

C．apart

D．up

呕血的常见病因是（）。

A.消化性溃疡

B.食管静脉曲张破裂出血

C.胃癌

D.急性胃粘膜病变

E.急性出血性胃炎