$D.D.T.$ is a/an:

Some toxic substances present in industrial waste water can undergo $.........$ in the aquatic food chain.

Why is $CNG$ considered a better fuel than diesel/petrol?
$I.$ $CNG$ burns most efficiently without leaving any unburnt remnant behind.
$II.$ $CNG$ is cheaper than petrol or diesel.
$III.$ $CNG$ cannot be siphoned off by thieves and adulterated like petrol or diesel.
Which of the statements given above are correct?

Compared to normal water,the $BOD$ of sewage-polluted water is . . . . . . .

Euro $II$ norms stipulate that sulphur be controlled at . . . . . . $ppm$ in diesel and . . . . . . $ppm$ in petrol.

What does '$Z$' represent in the given figure?