occur - happen; take place, crop - a cultivated plant that is grown on a large scale commercially, especially a cereal, fruit, or vegetable., fossil - the remains or impression of a prehistoric plant or animal embedded in rock and preserved in petrified form., contaminate - make (something) impure by exposure to or addition of a poisonous or polluting substance, chemical - a distinct compound or substance, especially one which has been artificially prepared or purified., exhaust - waste gases or air expelled from an engine, turbine, or other machine in the course of its operation., acid - a substance with particular chemical properties including turning litmus red, neutralizing alkalis, and dissolving some metals; typically, a corrosive or sour-tasting liquid of this kind., harness - control and make use of (natural resources), especially to produce energy., raise - to take care of a person, or an animal or plant, until they are completely grown, fertilizer - a chemical or natural substance added to soil or land to increase its fertility., pesticide - a substance used for destroying insects or other organisms harmful to cultivated plants or to animals., sewage - waste matter such as water or human urine or solid waste, soil erosion - the displacement of the upper layer of soil, depletion - reduction in the number or quantity of something,

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