Anna is working on finding the patterns of demographic variation within north america’s populations.  what is anna working on? a.  population structureb.  population sizec.  population densityd.  population growth

Quick she's working on population structure. Demographics is the study of  population variation. It is almost redundant.

It is not size, because she's finding how populations change with location and what influences that change (like water).

Population density is an entirely different topic. That tells how many people live in a unit of measurement. Usually is the number of people per square mile or square km.

Again growth is a different idea. This describes how fast something is increasing or decreasing.

the answer is:

carbon dioxide