A 9-year study was carried out on the pollutants released when biomass fuels are used for cooking. Researchers offered nearly 1000 people living in 12 villages in southern China access to clean biogas and to improved kitchen ventilation. Some people took advantage of neither; some changed to clean fuels; some improved their kitchen ventilation; and some did both. The following diagram shows data on the concentrations of nitrogen dioxide in these people’s homes at the end of the study.

Study the data represented in the diagram and then write a brief analysis that summarises the results of this part of the study.

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Groups: Neither Clean fuels only Nitrogen dioxide poluiant coneentration (mg Im² ) |Ventilation only O Both relermer ner 0.25 opy