4.

Develop circular models, with positive and negative feedback loops, for the following situations:

(a)
In a poor third-world country, the population increase has been very large in the last ten years. The river delta region has now become completely over-populated. A cyclone devastates the river delta region causing massive flooding, resulting in the deaths of 300,000 people.
(b)
A single farmer settles on a fertile uninhabited plain. After forty years, much of the plain has been transformed into a city.
(c)
A raindrop falls into a crack in the mountains. After fifty years, the mountain has become wooded.