Population is the total number of

The following diagrams represent the age pyramids of different populations. Comment on the status of these populations.

Population size keeps changing depending on different factor$(s)$ such as:

$(a)$ Label the three tiers $1, 2, 3$ given in the above age pyramid.
$(b)$ What type of population growth is represented by the above age pyramid?

Under a particular set of selection pressure,organisms evolve towards the most efficient reproductive strategy. For example:
$A$) Oysters produce a large number of small-sized offspring.
$B$) Mammals produce a small number of large-sized offspring.
In the light of the above statements,choose the correct option.

The bio-index number means