became the world's largest single source of air pollution by the 1990s.
a. auto emissions
b. waste incineration
c. metal smelting
d. chemical emissions

The answer is C. Auto Emissions

Explanation:

In the 1990's cars were the new thing of the time and since they were older models they didn't and couldn't properly dispose of gasoline in a gas form that polluted the air until newer versions came out that consumed less gasoline and produced less pollution.

its many islands, high mountains, and remote peninsulas isolated population centers.

the results of roman expansion produced social conflict and civil war. julius caesar gained power and became a dictator but was then assassinated. the reign of augustus began a long period of imperial rule and peace in the roman empire. rome faced the problems of how to maintain peace, law, and order.

hope this !

brainliest?