Concept:The upper part of the Earth’s crust is called “sial” because it is composed mainly of silica and alumina.Explanation:The word “sial” is formed from the first letters of its two chief constituents: “si” from silica and “al” from alumina.Silica is SiO2, and alumina is Al2O3.This granitic layer is lighter and forms the continental crust.Hence, the mineral composition of the sial points directly to silica and alumina.Answer:Option C: silica and alumina