Question

Which is the independent variable in the graph of the U.S. population from 1800 to 1900?

Answer 1

Answer: it is the year.

Explanation: the year is the independent variable because it is not caused by other variable, on the contrary, the population depend on the year that's why it is a dependent variable.

Answer 2

The amount of people in the U.S. depended on the year.  So the year is the independent variable.