How do you decide which is the independent variable for a scatter plot?

I'm given two sets of data and i need to make a scatter plot and i have to decide which is the independent variable but don't know which one to put. one set is the carbon emisions for year 2000 for 30 countries, the other is the gross national income for year 2000 for them 30 countries. I'm also asked how i decided which was the independent variable. thanks.How do you decide which is the independent variable for a scatter plot?
It's more likely that changes in GNI will affect the carbon emissions, so GNI is the independent variable and carbon emissions is dependent (on GNI).



As GNI goes up people will be doing more things that produce carbon emissions, like driving and burning fossil fuels for electricity. Fairly simple to link cause and effect.



In reverse, it's not likely that carbon emissions would change without a change in GNI and it would involve a long and complicated set of circumstances for GNI to be altered by changes in carbon emissions.