It is no_______ the attacks on the dollar have been led by countries that are politically opposed to the US.[ ]

A. coincidence    

B. chance    

C. purpose  

D. account

护背距离是指（）作用面至护轨头部外侧（作用边）的距离为护背距离。

A、基本轨

B、辙叉翼轨

C、护轨

D、尖轨