What statement best describes the purpose of the article “trash management”? A. to describe how to manage trash
b. to emphasize the problems of trash management
c. to list ways to solve waste management issues
d. to show how to reduce waste

Supporting details, comparisons to other diseases, countries, and possible treatment.  Central idea, often the main idea, needs factual or specific evidence to support, which is how you expand the idea.

In order to expand a central idea, you must use information that is specific to the main idea.  In this case, it was malaria being a deadly disease.  It is expanded upon by talking about the specific time period and how this affected treatment thereby being spread in the air.