移动式和携带式X射线机必须配备能阻止使用焦皮距小于（）cm的装置。
A.10
B.20
C.30
D.40
参考答案：B
在平面直角坐标系中，点P（a，b）在第二象限，则点P，（﹣a，b）在[ ]
A．第一象限
B．第二象限
C．第三象限
D．第四象限
_____ the website of the Fire Department in your city, and you will learn a lot about firefighting. [ ]
A. Search
B. To search
C. Searching
D. Having searched
