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Biological magnification, also known as bio-magnification refers to the increase in the concentration of toxic substances like DDT, chemicals, heavy substances etc. with each successive trophic level of the food chain. The concentration of these toxic substances is highest at the highest trophic level.

Lets take the example of DDT which have been applied on the farm lands. The DDT gets washed oof from the farm land reaches to the water. In water, the small phytoplanktons (Producers) would accumulate certain amounts of DDT in them and then it would be passed on to the next trophic level zooplanktons and then to (fishes) which feed on these zooplanktons. Concentration of DDT would increase with the trophic level. Producers having the minimum concentration while the tertiary consumers would have the highest concentration of DDT accumulation in them because they consume many secondary consumers which in turn consume many primary consumers . This can be illustrated by the following diagram:

29. The correct option is 3. Teeth.
Fluoride pollution causes decolourization of teeth, mainly turns the colour of teeth into yellow.

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