Fill in the blanks:
$(1)$ Hydrocarbon compounds are made from ........ and ........ .
$(2)$ ........ has a capacity to stop the flow of oxygen.
$(3)$ ........ is formed by attaching carbon monoxide with hemoglobin in blood.

The $COD$ (Chemical Oxygen Demand) of a water sample is determined using:

Which of the following is responsible for photochemical smog?

London smog is found in

The concentration of dissolved oxygen in water for the growth of fish should be more than $X \ ppm$ and the Biochemical Oxygen Demand $(BOD)$ in clean water should be less than $Y \ ppm$. $X$ and $Y$ in $ppm$ are,respectively:

The acid that is believed to be mainly responsible for the damage of the Taj Mahal is ...