Concept:Areas with conditions that support farming and settlement tend to attract more people.
Explanation:High population density occurs where many people can live comfortably.
Fertile soil supports agriculture and food production.
This attracts farmers and encourages permanent settlement.
It also leads to economic activities that draw more people.
Unemployment, unfavorable climate, and insects discourage dense settlement.
Therefore, fertile soil is a positive reason for high population density.
Answer:A. fertile soil