The author implies that corundum would be used to produce aluminum if A. corundum could be found that is not contaminated by silicates B. the production of alumina could be eliminated as an intermediate step in manufacturing aluminum C. many large deposits of very high quality corundum were to be discovered D. new technologies were to make it possible to convert corundum to a silicate E. manufacturers were to realize that the world's supply of bauxite is not unlimited