Questions

Because the refractive index of glass is highest, light travels slowly in Glass. 

We know that the speed of light depends on the refraction index. The higher the index of refraction is, the slower the speed of light is.

R.I of air and vacuum = 0 (air is a teeny bit more but can be taken as 0)
R.I of water = 1.33
R.I of glass = 1.4.

Hence, Light travels slowest in Glass.

In purifying drinking water alum is used for coagulation of mud particles.

National Income is the total income value of the goods and services produced by the residents of a country, usually during the period of one year.

National Income is categorized into 6 components. They are :

1. Compensation of Employees (Wages and Benefits)
2. Corporate Profits
3. Rental Income
4. Proprietors Income (Income of unincorporated business)
5. Net Interest

The Largest Component of National Income is Compensation of Employees.

The Largest component of National Income in India is Service Sector.

DPI [Dots Per Inch] is a measurement of output quality of printer resolution. It represents number of ink dots the printer can place in an area of one square inch. The lesser the DPI the less sharper the image and vice versa.

Therefore, a printer that may have 400 DPI, a printer that can print 400 x 400 per square inch or 1,60,000 dots per inch.