In an area where DDT had been used extensively, the population of birds declined significantly because

Correct Option (d) Reason : DDT, its breakdown products and other chlorinated hydrocarbon pesticides pose serious threat to birds. These persistant poisons enter the food chain and they accumulate in the fatty tissues of organisms at lower trophic level and then tend to concentrate in considerably toxic amount as they move up through the food chain. This is called biomagnification or bioconcentration. It weakens the calcarious egg shell of birds as it becomes very thin.