(i) Smog is a mixture of smoke, gases and chemicals.
(ii) It causes a smoky dark atmosphere, especially over cities.
(iii) It decreases visibility and creates haze throughout the area.

Population change refers to an increase or decrease in the population of an area influenced by the number of births, deaths and migration.

(i) In ancient Greek, 'demos' means people and 'graphis' means study of measurement.
(ii) So 'Demography' is the statistical study of human population.

(i) Many people move to urban areas primarily because cities promise more jobs better schools for poor's children and diverse income opportunities then subsistence forming in rural areas.
(ii) Urbanization drives economic growth and causes people to seek working and investment opportunities in urban areas.
(iii) Local governments are unable to manage urbanisation and migrant workers without an affordable place to live in, dwell in slums.
(iv) The VN.Habitat reports that 43% of urban population in developing countries and 78% of those in the least developed countries are slum dwellers.
(v) Lack of affordable low cost housing and poor planning encourages the supply side of slums.
(vi) Some of the slums in to-days world are a product of urbanisation brought by colonialism.
(vii) Urban poverty, encourages the formation and demand for slums.
(viii) Major natural disasters in poor nations often lead to migration of disaster affected form his to unaffiliated areas, the creation of temporary tent city and slums.

(i) The green house effect is the process by which radiation from a planets, atmosphere warms the planets' surface to a temperature above what it would be without its atmosphere.
(ii) If a planet's atmosphere contains radiatively active gases they will radiate energy in all directions.

(i) Natural Disaster or man made disaster such as famine, war, land slides, earthquake, Tsunami, epidemics, extreme weather, etc.
(ii) The black death is estimated to have killed 30 - 60 percent of Europe's total population during the 14^th century.
(iii) The dominant explanation for black death is attributed to the break of plague.

(i) Fracking not only pollutes water and air but also pollutes the soil.
(ii) The oil spills during fracking can harm the soil and the surrounding vegetation.
(iii) The use of high pressure at the time of soil extraction and the storage of waste water on site may cause earthquakes.

"Sustainable development is development that meets the needs of the present without compromising the ability of future generation to meet their own needs".

(i) The population growth increase when there are more births and immigration.
(ii) Unemployment, Social defence, Deforestation and Urbanisation are the mandate of population growth.

(i) Acid rain has many ecological effect, but none is greater than its impact on lakes, streams, wetlands and other aquatic environments.
(ii) Acid rain makes water acidic, and causes them to absorb the aluminium that makes its way from soil into lakes and streams.
(iii) It limits the nutrients available to the plants and expose them to toxic substances that are released from soil.

(i) Planting new trees can help to reduce the amount of CO₂ in the air.
(ii) Gases like Carbondioxide and methane are major contributors to the changing climate.
(iii) Reforestation is an effective mitigation strategy to fight global warming.
(iv) Reforestation can help to restore what the erosion has damaged.

Patterns of rural settlements: 

(i) Rectangular pattern: Rectangular pattern of settlements are found in plain areas or valleys. The roads are rectangular and cut each other at right angles.
(ii) Linear pattern : In a linear pattern, the houses are located along a road, railway line and along the edge of the river valley.
(iii) Circular or Semicircular pattern : The pattern of settlement that is found around the lakes, ponds and sea coasts are called circular or semicircular pattern.
(iv) Star like pattern: Where several metalled or unmetalled roads converge, star shaped settlements develop.
(v) Triangular pattern: Triangular patterns of rural settlement generally develop at the confluence of rivers
(vi) T-Shaped, V-Shaped, Cross-Shaped or Cruciform settlements: T-shaped settlements develop at tri-junctions of the roads (T) while V-shaped where two roads converge with the third one and Cruciform settlements develop on the cross-roads which extend in all your directions.
(vii) Nebular pattern: The arrangement of roads is almost circular which ends at the central location or nucleus of the settlement around the house of the main landlord of the village or around a mosque, temple or church.

Population distribution refers to the way in which people are spread out across the earth surface.
The world population is not uniformly distributed, owing to the following factors.
(a) Physical factors: It includes temperature, rainfall, Soil, relief, water, natural vegetation, distribution of minerals and availability of energy resources.
(b) Historical factors : Regions with historical importance (river valley civilizations, war and constant invasions fall under historical factors responsible for population distribution.
(c) Economic factors: Educational Institutions, employment opportunities, manufacturing industries, luxurious amenities, trade and commerce and other facilities encourage dense population in an area.

(A) and (R) are correct, (R) explain (A)

A and R are correct and A explain R