题干
Astronaut Zhai Zhigang has become the first Chinese_______in space.
A：walking
B：walked
C：to have walked
D：to walk
答案(点此获取答案解析）
D
A：walking
B：walked
C：to have walked
D：to walk
如图，圆弧形桥拱的跨度AB=12米，拱高CD=4米，则拱桥的半径为 ( ) 