Answer: Option (a) is the correct answer.
Explanation:
When rain passes through the air then carbon dioxide present in the air dissolves in the water. Further, this carbon dioxide reacts with water and results in the formation of carbonic acid.
Thus, the rain water becomes acidic in nature and as a result, there will be decrease in its pH.
Hence, we can conclude that rain has a natural pH that is acidic.
Every 4 years is a leap year 2016 is a leap year and so is 2020
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1) 
The capacitance of a parallel-plate capacitor is given by:

where
is the vacuum permittivity
A is the area of each plate
d is the distance between the plates
Here, the radius of each plate is

so the area is

While the separation between the plates is

So the capacitance is

And now we can find the energy stored,which is given by:

2) 0.71 J/m^3
The magnitude of the electric field is given by

and the energy density of the electric field is given by

and using
, we find
