President Hu' s visit to Australia, ___ first by a Chinese president in 12 years, is considered of ___ great importance to global economic situation at present.[ ]

A. a; \

B. the ; a

C. the; the

D. the; \

控制图中有三条线，中间一条线为中心线，上、下两条线分别为上、下控制线，这两条线距中心线的距离等于（）。

A．6σ

B．4σ

C．3σ

D．2σ