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The earth formed as a solid planet from the solar nebula about 4.6 million years ago. The most common element in the earth's atmosphere is oxygen. The lithosphere consists of the earth's crust and upper mantle. Circle the letter of the word or phrase that best completes the sentence or answers the question. The earth's crust is mostly made of A. magnesium silicates B. iron oxides C. aluminum silicates D. sodium hydroxides 6.