Evolution of the earth

Water and favourable atmosphere were necessary for the biosphere. Exam tip: link biosphere with life-supporting conditions.

Earth’s differentiation produced three broad compositional layers: the crust, the mantle and the core. In the hot early Earth, gravity caused dense metallic materials, mainly iron and nickel, to sink toward the centre and form the core. Less-dense rocky material formed the mantle and the thin outer crust. Consequently, option A is the only answer that names internal geological layers created through differentiation. Clouds, rain and wind are atmospheric phenomena; rivers, lakes and ponds are surface-water features; and grass, shrubs and trees are living organisms. None of these groups represents Earth’s internal structure.

The crust is the Earth’s outermost, solid and comparatively thin layer. Continents and ocean floors form part of this layer. The gaseous envelope is the atmosphere, so option C is not the crust; the central metallic part is the core. Exam tip: remember the order of Earth’s main layers from outside inward—crust, mantle, core.

Ammonia and methane were linked with the oxygen-poor early atmosphere. Exam tip: treat them as primitive atmospheric gases.

The hydrosphere means all the water found on Earth, including oceans, seas, rivers, lakes, groundwater, and water vapour. In the early stage, Earth was extremely hot, and water existed mainly as vapour. As the surface and atmosphere cooled, this vapour changed into liquid water. Therefore, the formation of the hydrosphere is mainly connected with condensation and rainfall, so option D is correct. Breaking stars, expansion of the Sun, and disappearance of the magnetic field do not explain the formation of Earth's oceans and other water bodies.

The sequence is important: water vapour first accumulated, cooling caused condensation, and continued condensation produced clouds and rainfall. Over a long period, rainwater collected in low-lying areas and helped form oceans. This does not mean that every part of the hydrosphere formed in exactly the same way, but condensation and precipitation are the central processes described in this question. Thus, option D gives the scientifically appropriate explanation.

As the surface cooled rainwater accumulated to form permanent water bodies. Exam tip: remember cooling with water storage.

Oceans provided a medium for chemical reactions and early life. Exam tip: connect seas with early life.

Increase in oxygen improved conditions for more complex life. Exam tip: link oxygen with biological evolution.

The early atmosphere had very little free oxygen. Exam tip: remember oxygen difference when comparing with today's atmosphere.

As the early Earth lost heat, its outermost material cooled and solidified, forming the first crust. This was the initial major step from a largely hot, molten body toward the differentiated solid planet that exists today. Option D is correct because surface cooling created a stable solid exterior, even though the deeper interior remained very hot and partly molten for a long time. Human agriculture, modern countries and books appeared billions of years later and are cultural developments, not stages in planetary formation. The key sequence is cooling, solidification and crust formation, followed by later atmospheric and hydrological changes.

The secondary atmosphere came from internal gases and water vapour. Exam tip: keep the sources of first and second atmosphere separate.

The ozone layer is a protective region of the atmosphere that absorbs much of the Sun’s harmful ultraviolet radiation. Ultraviolet radiation can damage living cells and make survival more difficult. The development of this protective layer therefore made Earth’s surface safer and supported the continuation and spread of life, especially after organisms began living outside water. It was one important environmental change in Earth’s long evolution.

Option B is correct because the formation of the ozone layer reduced exposure to harmful radiation. The disappearance of all oceans would remove an essential habitat and water source, not improve safety. The Moon cannot turn into the Sun, and gravity is necessary for Earth to hold its atmosphere and maintain stable conditions. Thus the ozone layer is the only scientifically suitable choice among the options.

Liquid water provided a medium for chemical reactions. Exam tip: link liquid water with pre-life chemistry.

Before oceans formed water was in the atmosphere as vapour. Exam tip: remember vapour to rain to oceans.

Earth was hot first then cooled to form crust and oceans. Exam tip: arrange stages chronologically in sequence questions.

Extreme heat and unstable surface were not favourable for life. Exam tip: remember harsh conditions before life began.

Water vapour condensed into rain and then formed oceans. Exam tip: treat water vapour as the beginning of hydrosphere.

Photosynthetic organisms increased oxygen in the atmosphere. Exam tip: remember them as biological agents of atmospheric change.

Earth had to cool sufficiently for life. Exam tip: understand temperature as a basic condition for life.

The formation of oceans helped moderate and stabilize conditions at Earth’s surface. Water has a high heat capacity, so it can absorb, store and redistribute heat; ocean circulation also moves energy between regions. In the early Earth, the development of liquid water therefore reduced some temperature extremes and supported a more suitable environment for continuing geological and biological change. Option A is correct because it states this general balancing effect without claiming that oceans made temperature perfectly constant. Oceans did not turn Earth into a star, remove every atmospheric gas, or bring the core to the surface; options B, C and D contradict basic planetary science.

A famous idea says Moon formed from material ejected after a large impact. Exam tip: link it with the giant impact idea.

Natural systems on Earth developed gradually. Exam tip: treat these spheres as stages of Earth's evolution.

Lack of oxygen shows the primitive atmosphere was not like today's. Exam tip: compare primitive and modern atmospheres.

Liquid water and suitable gases provided the basis for life. Exam tip: remember water and gases as two elements of a life-supporting environment.

Earth's major systems formed over a very long time. Exam tip: treat Earth's evolution as gradual and long-term.