空中加油

中文版：

飞机A和飞机B，加满油都可以在空中飞行6000千米。若A和B加满油后同时从C机场起飞，飞机内没有备用油箱。A在空中某处给B“空中加油”，那么飞机B最远可以飞到离C机场多少千米的地方就要返回，才能确保飞机A和B都能安全返回C机场？

 英文版：

Both aeroplane A and aeroplane B can fly for 6000 kilometers when they have a full tank of fuel. If both of the planes depart from Airport C with a full tank and aeroplane A can refuel aeroplane B from its own tank at some point in the air. In order to make sure both aeroplane A and B can return to Airport C before their fuel run out, what is longest distance aeroplane B can reach (in kilometers)?