When the Earth cooled from being hot, all the iron and nickel was concentrated inside the Earth forming the inner two layers, inner solid core and outer liquid core. Then with the passage of time the Earth solidified gradually and the mantle solidified and now at this stage the Earth's crust, the outer most part of the Earth, looks like it's final product (the way it does now).
The anser is D: compound. A compound sentence is when two or moreindependent clauses that have related ideas are joined with a coordenator (for, and or but) or a semicolon. Independent clauses can stand alone as a complete thought. They are not dependent of one another to express a complete thought, but they join similar ideas.
