What happens when nitrates and phosphates are added to a lake?

In an aquatic food chain,the highest concentration of $DDT$ is found in which of the following?

Catalytic converters:
$I.$ These are fitted into automobiles for reducing emission of poisonous gases like $NO_x$ and $CO$.
$II.$ They have expensive metals like platinum,palladium,and rhodium as catalysts.
$III.$ As the exhaust emission passes through the catalytic converter,nitric oxide splits into nitrogen and oxygen; carbon monoxide is oxidized to carbon dioxide,and unburnt hydrocarbons are burnt completely into $CO_2$ and $H_2O$.
$IV.$ Motor vehicles fitted with catalytic converters should use unleaded petrol because lead in the petrol inactivates the catalyst.
Which of the statements given above are correct about catalytic converters?

In the last few decades,the most serious nuclear reactor accident and the $MIC$ gas tragedy occurred in:

What is the major drawback of $DDT$ as an insecticide?

Thermal pollution of water bodies is due to