Which of the following metals causes harmful effects?

$DDT$ residues are rapidly passed through food chains causing biomagnification. What is the reason for this?

Which metal is the most harmful pollutant emitted by automobiles?

When does the $BOD$ (Biochemical Oxygen Demand) decrease?

Which of the following problems is created by a brief exposure to an extremely high sound level,$150 \, dB$ or more,generated by the takeoff of a jet plane or rocket?

In a eutrophic water body,the excessive growth of $\underline {X}$ leads to the death of $\underline {Y}$.