Early and modern theories

Heavy materials such as iron and nickel moved toward the center due to density. In exams, connect metals and density with differentiation.

The governing concept is the thermal evolution and internal differentiation of the early Earth. During its formation, repeated impacts by planetesimals and meteorite-like bodies converted kinetic energy into heat. Radioactive decay inside the young planet supplied an additional internal heat source. Together, these processes helped keep the early Earth partly molten or highly heated for a long period. In such conditions, denser materials such as iron could move downward while lighter silicate materials rose, creating the possibility of layered internal structure. Option A is correct because it states both the heating effect and its consequence for differentiation. Option B is unrelated and much too immediate, while options C and D do not follow from these heat-producing processes.

The molten state allowed heavy and light materials to separate. In exams, treat melting as a condition for layer formation.

Gas release brought out water vapour and other gases. In exams, connect outgassing with atmosphere formation.

Free oxygen increased later through biological processes. In exams, do not treat the early atmosphere like the present one.

The hydrosphere means all the water found on Earth, including oceans, rivers, lakes, groundwater and water vapour. In the early stage, Earth was extremely hot, so water could remain mainly as vapour. As the surface and atmosphere cooled, the vapour could change into liquid water. This basic connection between cooling, condensation and the formation of water bodies is the central idea.

The correct sequence is that Earth cooled first, water vapour condensed next, and the resulting water fell as rain and collected in low areas to form oceans, lakes and other reservoirs. Therefore, option A gives the complete and logical chain. Option B reverses the order of important events, while the red shift and steady-state ideas do not explain hydrosphere formation. The supplied answer is consistent.

The solid crust provided a base for hydrosphere and surface development. In exams, connect crust with water retention.

The early atmosphere had little free oxygen and it increased later. In exams, treat atmospheric evolution as gradual.

The giant impact idea links Moon formation with large impacts. In exams, connect it with the active state of early Earth.

Radioactive decay rates give a scientific basis for age estimation. In exams, connect Earth's age with an evidence-based method.

Big Bang is at universe scale, solar nebula at solar-system scale, and differentiation at Earth scale. In exams, identify scale.

A passing star and matter pulled from the Sun are clues of the tidal theory. In exams, identify theories through clue words.

These clues identify the modern solar nebula model. In exams, identify gas-dust cloud and disk together.

Earth's internal layers are explained by differentiation, not only by the Big Bang. In exams, write the process at the correct scale.

A planet first forms through physical processes, and life develops later. In exams, do not mix the two stages.

Different Earth spheres developed through different processes. In exams, match process and sphere correctly.

New evidence and better physical understanding improve theories. In exams, treat science as evidence-based and changeable.

Modern theories are more evidence-based and broadly explanatory. In exams, clearly state the basis while comparing.

The old nebular model could not fully explain some physical distributions. In exams, remember the limitation with meaning.

Red shift and background radiation are major supports for the Big Bang. In exams, connect evidence with the correct theory.

The modern solar nebula model explains many solar-system observations together. In exams, write the model with its reason.

Lighter silicate materials remained above and helped develop mantle and crust. In exams, connect core, mantle, and crust with density-based differentiation.

The Big Bang explains the beginning of the universe, not Earth's direct formation. In exams, identify the scale of the theory first.

Gravitational collapse concentrated matter at the center and left the remaining material in a disk. In exams, write the separate roles of center and disk.

Red shift indicates galaxies moving away. In exams, write the reasoning from evidence to conclusion.