主动脉夹层常见的易患因素不包括（）
A.妊娠
B.动脉粥样硬化
C.Marfan综合征
D.低血压
E.二叶主动脉瓣
参考答案：D
_________ is a growing world problem, and nowadays the United Nations and its member countries _________hard to deal with it．
A．Desertification; are working
B．Desertified ; worked
C．Desert; work
D．Desertification; have worked
课堂气氛也会使许多学生追求某种行为方式，从而导致学生间发生( )。
A．连锁性感染 B．互相监督C．互相帮助 D．遵守纪律
