Under natural conditions, oceans are
self-cleaning systems
ideal waste-dumping grounds
of no use to humans
most damaged by air pollution

Self-cleaning systems

Explanation:

Oceans are natural self cleaning systems that can get rid of any material on their own.

At some parts of the ocean, new crust are being formed. In some other part, new crustal materials are added.

When organisms in the ocean die, they decay to produce some carbon compounds which goes back to the ocean and atmosphere. Their skeletal parts goes to the bottom of the ocean where it compacts with sediments to form new sedimentary rock sequences.

When wastes from land are brought into the ocean, they can be broken down on the long run.

The ocean is self cleaning and it is an effective system as such.

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]