The industrial pollutants known as PCB's were first manufactured in 1929 and were used as coolants for electrical equipment in Europe and North America the 1970s, when studies showed that the compounds had entered the food chain and were harmful to some animals.
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Athat the compounds had entered the food chain and were harmful to some animals
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Bthe compounds to have entered the food chain and be harmful to some animals
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Cthe entry of the compounds into the food chain as harmful to some animals
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Dthe entry of the compounds into the food chain and its harmfulness to animals
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Ethe compounds entering into the food chain and harming some animals
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