Fill in the blanks:
$(1)$ The types of smog are ..... and ..... .
$(2)$ Ozone in the air reacts with hydrocarbons to form ...... and ...... .
$(3)$ Harmful ultraviolet rays from the sun cause a type of skin cancer known as ...... .
$(4)$ Chlorofluorocarbon compounds are also called .......... .

During a school field trip,a botany student visited a beautiful pond in a village and collected some plants. He observed that villagers were washing clothes there,and household waste was spoiling its beauty in some areas. After a few years,he revisited the pond and found it completely covered with algae,emitting a foul smell,and the water had become unusable. Can you describe this condition of the pond?

Biochemical Oxygen Demand $(BOD)$ is the amount of oxygen required (in $ppm$):

The presence of soluble fluoride ion up to $1 \ ppm$ concentration in drinking water is:

The enamel present on teeth becomes much harder due to the conversion of $[3 Ca_3(PO_4)_2, X]$ into $[3 Ca_3(PO_4)_2, Y]$. What are $X$ and $Y$?

According to current information,what is the primary cause of atmospheric pollution in industrial cities leading to acid rain?