Why are the world population concentrated on the continental margins

Most of the population of the world is concentrated on the continental margins, or more specifically, along the coastlines. This is due to multiple factors, some of which are:

- Climate;

The climate in the coastal regions is usually much more pleasant that the climate deeper into the mainland, so naturally the people preferred to live where it is more pleasant.

- Arable land;

In close proximity to the coastline there's usually nice fertile soil, so it is very suitable for production of food, thus managing to sustain yourself.

- Economy;

The coastal regions, in lots of cases, are well suited for building ports, and that makes them excellent places from where trade can take place, so it strengthens the economy of these places.

a. i. only is correct.