—What ______ we do then?
—Let’s go out for a walk.[ ]
A. do
B. must
C. shall
D. did
答案:C
若全集U={1,2,3,4,5},CUA={1,3},则A=______.
下列哪些是主动脉瓣关闭不全的病因()
A.风湿性心脏病
B.主动脉夹层动脉瘤
C.马方综合征
D.细菌性心内膜炎
E.先天性主动脉瓣畸形