TOEFL Reading: ETS-TOEFL阅读机经 - QY457KJ53X6C6WX5L$

Select the TWO answer choices that, according to paragraph 3, indicate true statements about Earth's formation. To obtain credit, you must select TWO answer choices. A. Comets hitting Earth helped the lightest silicates to reach Earth's surface. B. Silicates such as mineral quartz drifted downward and mixed with denser materials as they reached Earth's core. C. When Earth's mantle became approximately 3,000 kilometers thick, the heaviest materials in it began to cool. D. Lighter materials reaching Earth's surface formed Earth's continental crust.