Answer: The correct option is 2.
Explanation: Polyatomic ions are defined as the ions in which two or more elements are covalently bonded to act as a single unit. They always carry some charge.
Examples of polyatomic ions are: 
known as hydroxide ion, 
known as sulfate ion
We are given a compound 
having 2 ions which are 
and 
. these ions are named as sodium ions and hydrogen carbonate ions.
Therefore, the correct option is 2.
Not sure about the first two but I think number three is B. sorry if I'm wrong
Answer:- The correct answer is B. measuring the height of a building.
Explanations:- Insects and ants are very small. So, meter is not the right unit of measure their length. Their lengths would be measure in mm that is millimeters. So, options A and C are not good.
The distance between two cities is mostly too much so it's measured in kilometers or miles. So, D is also not good.
Height of buildings is measured in meters and so the correct option is B.
A layer above the earth that protects the planet from dangerous UV that are sent from the sun
