拱式桥在竖向荷载作用下，两拱脚处不仅产生竖向反力，还产生（）。
A．扭矩
B．水平推力
C．剪力
D．拉力
参考答案：B
His dog did everything he told it to do. ___ a most obedient animal.
A．It is
B．It were
C．It are
D．It was
（）是属于按推进动力分类的船舶
A.螺旋桨船
B.平旋推进器船
C.喷水推进船
D.电力推进船
