Toss Up - Earth And Space - Multiple Choice

Within the early Earth's vast molten region, substances underwent a process known as differentiation, during which
W) substanes of low density rise to Earth's surface, while those of high density sink toward its center
X) substances of high density float to Earth's surface, while those of low density sink toward its center.
Y) substances of high and low density chemically combine to form uniformly dense substances.
Z) substances of high density form gases, while those of low density form solids.