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Question 11 (5 marks) A river flows through a region consists of forest, farm, and city areas as shown in Figure Q11. Every year, the farmers put fertilizer on some of the fields. When there is a lot of rain, some of the fertilizers is washed into the river. 401 240******0 OFE Lake 5 km Key Forest Direction of water flow Farm City Figure Q11. A map showing the position of the farm and a river that flows through it. a) When the fertilizer goes into the river, the concentration of dissolved oxygen (DO) in the water will decrease. Suggest a possible explanation for the decrease in DO (4 marks). b) There is a city -5 km downstream from the farm. Apart from fertilizer, suggest one other pollutant that might get into the river as it flows through the city (1 mark).