6. are areas on the ocean floor that experience active volcanism and are where plates are pulled apart from one another a) mid-ocean ridges b) subduction zones c) spreading centers d) upper mantle e) both a) & c) 7. the solar system initially flattened into a due to the conservation of angular momentum. a) ball b) sphere c) disk d) doughnut 8. the planets are thought to have initial grown into planetesimals through a) accretion b) indentation c) condensation d) differentiation 9. which are the 2 most abundant elements in the universe? a) hydrogen and oxygen b) oxygen and helium c) oxygen and hydrogen d) hydrogen and helium 10. the physical property that describes an object's mass per unit volume is a) density b) buoyancy c) gravity d) weight 11. the underlying mechanism by which plate tectonics operates is a) differentiation b) accretion c) mantle convection d) inception