单选题
Ditrama is a federation made up of three autonomous regions: Korva, Mitro and Guadar. Under the federal revenuesharing plan, each region receives a share of federal revenues equal to the share of the total population of Ditrama residing in that region as shown by a yearly population survey. Last year, the percentage of federal revenues Korva received for its share decreased somewhat even though the population survey on which the revenue-sharing was based showed that Korva's population had increased. If the statements above are true, which one of the following must also have been shown by the population survey on which last year's revenue-sharing in Ditrama was based?