读图,回答问题。图中共有几级天体系统 ( )
地球是中心天体的天体系统是 ( )
下图中能正确表示河外星系、太阳系、地球三者关系的是 ( )
_____ three influenza type A virus subtypes(子类型) have been found in pigs.
_____, experts point out that students can easily fail at starting their business because of a lack of preparations.
It has always been the _____ of our goverment to encourage graduates to start their business.
The most important _____ of his speech at the G20 summit was that we should all unite to fight against the finanical.
He directed exposure to pigs,otherwise he_______pig flu.