A saturated geological unit which can yield water to the wells at a sufficient rate to support a well is called ______.

The correct answer is A, Unreliable supply. Most renewable sources depend on the weather, so the power they give is intermittent rather than steady. Solar panels make nothing at night and little under thick cloud, wind turbines stop when the wind drops, and hydropower falls away in a weak monsoon. Because electricity has to match demand minute by minute, this variability is the main engineering problem with renewables, and it is why grids need storage, backup plants or a wide mix of sources. Option B is wrong because costly waste disposal is a drawback of nuclear power, not of solar or wind. Option C is wrong because renewables are valued precisely for releasing almost no carbon dioxide or particulate pollution while they run. Option D is wrong because their running cost is very low, since sunlight and wind are free and the big expense is the one-time cost of installation. Exam tip: renewables have a high capital cost and an unreliable supply, but a low running cost and low pollution.

The correct answer is D, SDG 15. Sustainable Development Goal 15 is Life on Land, and target 15.3 commits countries to combat desertification, restore degraded land and soil including land affected by desertification, drought and floods, and strive to achieve a land degradation neutral world. Land Degradation Neutrality means that the amount of healthy and productive land stays stable or increases, because whatever is degraded is matched by an equal area restored. A is wrong because SDG 6 is Clean Water and Sanitation. B is wrong because SDG 13 is Climate Action, which is linked to land degradation through drought and carbon loss but does not carry the neutrality target. C is wrong because SDG 14 is Life Below Water and deals with oceans, seas and marine resources. Remembering the pair of goals 14 and 15 as water and land is the quickest way to avoid this trap.

The correct answer is D, Ramsar Convention on Wetlands. The Ramsar Convention was signed at Ramsar in Iran in 1971 and came into force in 1975, two decades before the Earth Summit, and its secretariat sits at Gland in Switzerland; it therefore has no connection with Rio. The three Rio Conventions are those that arose from the United Nations Conference on Environment and Development held at Rio de Janeiro in 1992. A is wrong as an answer because the UNFCCC was opened for signature at Rio in 1992 and entered into force in 1994. B is wrong as an answer because the Convention on Biological Diversity was also opened for signature at Rio in 1992 and entered into force in 1993. C is wrong as an answer because the UNCCD, although adopted at Paris in 1994, was mandated by Agenda 21 at Rio and is counted as the third Rio Convention.

The correct answer is C, Bonn. The permanent secretariat of the United Nations Convention to Combat Desertification has been based at Bonn in Germany since 1999, in the same United Nations campus that hosts the secretariat of the UNFCCC, which is a useful memory link because both are Rio Conventions with their secretariats in the same German city. A is wrong because Geneva hosts bodies such as the World Health Organization, the World Meteorological Organization, the World Trade Organization and the Intergovernmental Panel on Climate Change secretariat, but not the UNCCD. B is wrong because Nairobi is the headquarters of the United Nations Environment Programme and of UN-Habitat, a fact often mixed up with this one. D is wrong because Paris is where the Convention was adopted in 1994 and is the seat of UNESCO, but the Convention's working secretariat is not there.

The correct answer is C, Gully erosion. Running water in the Chambal basin of Madhya Pradesh, Rajasthan and Uttar Pradesh has cut deep, steep-sided channels into soft alluvium, and when these channels multiply and widen the land becomes unfit for cultivation. Such dissected and broken land is called badland or ravine land, and the Chambal ravines are the standard Indian example of gully erosion. A is wrong because sheet erosion removes a thin and even film of topsoil over a wide surface and leaves no visible channel, which is why it is the most insidious rather than the most visible form. B is wrong because rill erosion is the intermediate stage of small finger-like channels that can still be ploughed over; gullies are what rills become when they are not checked. D is wrong because wind erosion produces shifting sand dunes and deflation hollows, and dominates western Rajasthan rather than the Chambal ravines.