The object of this quiz is to click on the circle containing the largest population. When you begin you'll be shown 2 circles on the map with radii between 10 and 250 miles which do not intersect. Click on the one with the larger population within it (based on US census tract info) then 'Submit' - if you are correct you'll move on. One incorrect round and the quiz is over. Every 5 rounds the number of circles on the map will increase by 1. Try to build the longest streak of correct circles. Circles displayed will always be non-intersecting.
NOTE: You may have to zoom out to see all circles, occasionally a AK/HI circle will be included. Also, only population within the US is included in the total, any Canada/Mexico coverage counts as 0 population within the circle.
Quiz Takers 
Why do Davenport and Bettendorf, IA count but not Moline or Rock Island, IL across the river? Cape Girardeau seems to be missing as well.
It looks like for the cities not included the actual city border doesn’t extend into the river where the state border is. You can see this pulling up the cities on Google Maps.