What is the scientific study of human populations called?

How many persons are added to the Indian population per year?

At the time of independence,our population was approximately ........... billion,which reached near ............ billion in the year $2000$ and crossed ........... billion in $2011$.

In which of the following countries is the birth rate high and the death rate normal?

$Statement 1$: Over-population has become a serious problem in the developing countries.
$Statement 2$: It does not exhaust natural resources, causes unemployment and pollution.

Population explosion occurs due to