Which of the following statements with reference to Surface inversion of temperature is/are correct ? 1. It causes instability in the lower layers of the atmosphere. 2. This inversion commonly lasts for a few hours until the Sun comes up. Select the answer using the code given below :

The correct answer is C, 1971. The 1971 Census recorded a decadal growth of about 24.8 per cent for 1961–71, the highest of any decade in India's census history.

Indian censuses are taken every ten years, and the decadal growth rate compares each count with the one before. Growth was 21.6 per cent in the 1961 Census, rose to 24.8 per cent in 1971, then eased to 24.7 per cent in 1981 and 23.9 per cent in 1991, before falling to 21.5 per cent in 2001 and 17.7 per cent in 2011. The 1951–81 period is called the phase of population explosion, when death rates fell sharply with better health care while birth rates stayed high. The year 1921 is called the year of the great divide, the last census to record a fall in population.

A is wrong: the 1991 figure, about 23.9 per cent, was lower than in 1971.

B is wrong: 1981 recorded about 24.7 per cent, slightly below 1971.

D is wrong: 1961 recorded about 21.6 per cent.

Exam tip: highest decadal growth = 1971 Census (24.8%); 1921 = year of the great divide; 1951–81 = population explosion.

The correct answer is D, Tundra region. The tundra is too cold, with a very short growing season and permanently frozen subsoil, for trees to grow, so it has no timber.

The tundra lies north of the treeline, along the Arctic coasts of North America, Europe and Asia. Winters are long and severe, summers last only two or three months, and below a thin active layer the ground stays frozen as permafrost, which roots cannot penetrate. Vegetation is limited to mosses, lichens, sedges and dwarf shrubs. Reindeer, caribou and the Arctic fox live there. The word tundra comes from a Finnish term meaning treeless plain.

A is wrong: subtropical regions support evergreen and deciduous forests with valuable hardwoods.

B is wrong: temperate regions have broad-leaved and coniferous forests that are the world's major softwood sources.

C is wrong: the alpine zone has coniferous trees such as fir, spruce and birch on its lower slopes before the treeline.

Exam tip: taiga = coniferous softwood belt; tundra = treeless, mosses and lichens; equatorial forests = hardwood that is hard to log commercially.

The correct answer is C, Both 1 and 2. Humus is the dark, amorphous organic matter of soil, and its colloidal nature lets it hold nutrients and water for plants.

Humus forms when micro-organisms break down dead leaves, roots and animal remains in the soil. The end product is dark brown to black and has no definite shape or structure, which is what amorphous means, so statement 1 is correct. Humus particles are colloidal: they are extremely fine and carry a large, negatively charged surface. That surface holds cations such as calcium, magnesium and potassium and releases them slowly to plant roots, and it also retains moisture. This is why humus is called a storehouse or reservoir of nutrients, so statement 2 is correct. Soils rich in humus, such as forest and peaty soils, are dark in colour.

A is wrong: statement 2 is also correct.

B is wrong: statement 1 is also correct.

D is wrong: both statements describe humus accurately.

Exam tip: humus = dark, amorphous, colloidal organic matter; it improves soil structure, water retention and nutrient supply.

The correct answer is D, Bauxite. Bauxite is a mixture of hydrated aluminium oxides and hydroxides, not a silicate, and it is the main ore of aluminium.

Silicates are minerals built from silicon-oxygen tetrahedra, and they make up most of the earth's crust. Bauxite forms when rocks rich in aluminium are deeply weathered in hot, wet tropical climates, which washes the silica away and leaves aluminium oxide behind; minerals such as gibbsite and boehmite give it its value. In India it is mined mainly in Odisha, along with Gujarat, Jharkhand and Chhattisgarh.

Option A is wrong because feldspar, an aluminosilicate of potassium, sodium or calcium, is the most common mineral group in the crust and is used in ceramics and glass. Option B is wrong because zeolites are porous hydrated aluminosilicates used as water softeners and catalysts. Option C is wrong because mica is a sheet silicate that splits into thin layers and is valued as an electrical insulator; Jharkhand, Andhra Pradesh and Rajasthan are known for it.

Exam tip: feldspar, mica, zeolite = silicates; bauxite = aluminium oxide ore, Odisha leads.

The correct answer is A, 1 only. Only the Aral Sea has shrunk drastically because of human activity.

The Aral Sea, on the border of Kazakhstan and Uzbekistan, was once the world's fourth-largest lake. From the 1960s, Soviet planners diverted its two feeder rivers, the Amu Darya and the Syr Darya, to irrigate cotton fields, and the lake lost most of its area, splitting into smaller bodies and leaving a salty desert, the Aralkum. The Black Sea is a large inland sea joined to the Mediterranean through the Bosporus and has not dried up. Lake Baikal in Siberia, the world's deepest and oldest freshwater lake, faces pollution but has not shrunk away.

B is wrong: neither the Black Sea nor Lake Baikal has dried up.

C is wrong: the Black Sea has not shrunk.

D is wrong: Lake Baikal has not shrunk immensely.

Exam tip: Aral Sea = Amu Darya and Syr Darya diverted for cotton; Baikal = deepest lake in the world.