18. the three most common elements in the earth's crust, in order of decreasing abundance, are a. feldspar, quartz, pyroxenes b. silicon, oxygen, copper c. oxygen, silicon, aluminum d. iron, calcium, sodium

c. Oxygen, silicon, and aluminium

Explanation:

The common elements in the Earth's crust in order of decreasing abundance are as follows:  

Oxygen: Oxygen is the most abundant element on earth's crust. It makes up 46.6%. It exists as compound of the silicate minerals. It is used for industrial, commercial and medical purposes.

Silicon: Silicon is the second most common and abundant element on the earth crust. It makes up the concentration of 27.7%. It combines with the minerals of silica to form the silicate minerals. It is used for the purpose of ceramics, cosmetics, glass and other products.

Aluminium: Aluminium is the third most common and abundant element on the earth crust. It makes up the concentration of 8.1%. The aluminum exists in the form of aluminium oxide, potassium aluminium sulfate and aluminium hydroxide. It is used for the purpose of manufacture of rockets, machinery and automobiles.

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