The gas that leaked from the Union Carbide plant in the Bhopal Gas Tragedy of 1984 was:

The correct answer is A, 2-3 December 1984. The leak at the Union Carbide India Limited plant in Bhopal began late on 2 December 1984 and the gas spread through the early hours of 3 December, which is why India observes 2 December as National Pollution Control Day. Option B is wrong because 26 April 1986 is the date of the Chernobyl reactor explosion in the Ukrainian SSR, now marked by the United Nations as International Chernobyl Disaster Remembrance Day. Option C is wrong because 11 March 2011 is the date of the Tohoku earthquake and tsunami that flooded the Fukushima Daiichi nuclear plant in Japan. Option D is wrong because 10 July 1976 is the date of the dioxin release at Seveso in Italy, the accident behind the European Seveso Directive. Keeping these four dates apart is the whole exercise in this question.

The correct answer is C, Ukraine. Reactor Number 4 of the Chernobyl Nuclear Power Plant, near the town of Pripyat, exploded on 26 April 1986. The site then lay in the Ukrainian Soviet Socialist Republic of the USSR and lies in independent Ukraine today, which is why the question is usually framed as present-day country. Option B is wrong, although it is the most tempting distractor: Belarus lay downwind and received a large share of the radioactive fallout, but the plant itself was not on Belarusian soil. Option A is wrong because Russia, though the successor state of the Soviet Union, did not contain Chernobyl; the Soviet-era Kyshtym accident of 1957 at the Mayak plant is the Russian one. Option D is wrong because Kazakhstan is associated with the Semipalatinsk nuclear test site, not with Chernobyl.

The correct answer is B, mercury. The Chisso Corporation discharged methylmercury compounds into Minamata Bay in Kumamoto Prefecture; the mercury accumulated through fish and shellfish into the people who ate them and attacked the central nervous system, producing numbness, loss of coordination, blindness and congenital deformity. The disease gave its name to the Minamata Convention on Mercury, adopted at Kumamoto in 2013. Option A is wrong because cadmium causes Itai-itai disease in the Jinzu river basin of Toyama Prefecture, the standard partner question to this one. Option C is wrong because lead poisoning is called plumbism or saturnism and is linked with paints, pipes and battery work, not with Minamata. Option D is wrong because chronic arsenic poisoning from groundwater is a problem of West Bengal and Bangladesh, where it causes arsenicosis and black-foot disease.

The correct answer is A, cadmium. Mining waste carrying cadmium entered the Jinzu river in Toyama Prefecture and reached people through irrigated rice and drinking water. Cadmium damages the kidneys and softens bone, so that bones fractured under ordinary use; the Japanese name itai-itai is the cry of pain the sufferers made, and the condition is also described as osteomalacia with osteoporosis. Option B is wrong because excess fluoride causes fluorosis, with mottled teeth and bent limbs, a problem of parts of Rajasthan, Andhra Pradesh and Telangana in India. Option C is wrong because mercury is the agent of Minamata disease, the other Japanese pollution disease and the commonest confusion in this pair. Option D is wrong because nickel is associated with contact dermatitis and lung effects in refinery work, not with itai-itai. Remember the pair: mercury to Minamata, cadmium to itai-itai.

The correct answer is B, mercury. The Minamata Convention on Mercury was adopted at Kumamoto, Japan, in 2013 and entered into force in 2017. It is named after the Japanese town whose people were poisoned by industrial methylmercury, and it controls the supply, trade, use and emission of mercury, phasing out mercury mining and many mercury-added products. Option A is wrong because persistent organic pollutants are the subject of the Stockholm Convention of 2001, which covers the so-called dirty dozen chemicals. Option C is wrong because ozone-depleting substances fall under the Vienna Convention of 1985 and the Montreal Protocol of 1987. Option D is wrong because the transboundary movement of hazardous wastes is governed by the Basel Convention of 1989, adopted after rich countries were found dumping waste in poorer ones. Examiners regularly place these four treaties in one matching question.