旋翼机是从减轻飞机自身重量的角度进行设计制造的。

下列属于组合体系桥梁的是()。

A．钢管混凝土拱桥

B．斜拉桥

C．连续刚构桥

D．系杆拱

E．斜腿刚架

椎管狭窄症包括（）

A.椎管狭窄

B.椎间盘疝

C.侧隐窝狭窄

D.椎间孔狭窄

E.椎间盘变性

If they will not be able to reach agreement before the conference, they shall lose a good opportunity of involving themselves to do the project.

A．will be unable ... to involving

B．are unable ... to involve

C．are not be able ... to involve

D．will be able ... to involving

逆流塔的优缺点是什么？