The population of India at the time of Christ was approximately

Which of the following can be taken as characteristic demographic features of the developing countries?
$(a)$ High fertility rate
$(b)$ Rapidly rising mortality rate
$(c)$ Very young age distribution
$(d)$ Rapid population growth
$(e)$ Rapidly falling mortality rate
$(f)$ Very old age distribution

July $11$ is observed as ........

Which of the following developing countries has a high death rate?

The world population in $1900$ was ........ billion,which increased rapidly to ........ billion in $2000$ and reached ........ billion in $2011$.

The greatest biological problem faced by human beings is: