17. The author implies that a mineral must either be or readily supply which of the following in order to be classified as an aluminum ore?

　　An aggregate

　　Bauxite

　　Alumina

　　Corundum

　　An aluminum silicate

　　18. The passage supplies information for answering all of the following questions regarding aluminous minerals EXCEPT:

　　What percentage of the aluminum in the Earth s crust is in the form of bauxite?

　　Are aluminum-bearing nonbauxite minerals plentiful?

　　Do the aluminous minerals found in bauxite contain hydrated oxides?

　　Are aluminous hydrated oxides found in rocks?

　　Do large quantities of bauxite exist?

　　19. The author implies that corundum would be used to produce aluminum if

　　corundum could be found that is not contaminated by silicates

　　the production of alumina could be eliminated as an intermediate step in manufacturing aluminum

　　many large deposits of very high quality corundum were to be discovered

　　new technologies were to make it possible to convert corundum to a silicate

　　manufacturers were to realize that the world s supply of bauxite is not unlimited

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