What are mountains and plains ???​

Explanation:

In geography, a plain is a flat expanse of land that generally does not change much in elevation, and are primarily treeless. Plains occur as lowlands along valleys or at the base of mountains, as coastal plains, and as plateaus or uplands. ... In a few instances, deserts and rainforests may also be considered plains.

d. stage 4

explanation:

the existence of some kind of demographic transition is widely accepted in the social sciences because of the well-established historical correlation linking dropping fertility to social and economic development.[1] scholars debate whether industrialization and higher incomes lead to lower population, or whether lower populations lead to industrialization and higher incomes. scholars also debate to what extent various proposed and sometimes inter-related factors such as higher per capita income,lower mortality, old-age security, and rise of demand for human capital are involved.[2]