Per year growth in India's population according to the $1991$ census is (in $\%$)

Presently, the world population doubles after every (in $years$)

According to the $2001$ census,the Indian population was:

In the world population, how many people are added per year?

The $20^{th}$ century has witnessed a remarkable increase in the population of the world and especially in India. One major factor for this is that

What is the scientific study of human populations called?