I do not think either population or populations is a critical factor here. In any case, 'populations' is a legitimate plural form available in English and has been used conspicuously when a large number of diverse settings are to be described. Similarly, since, the 'cities' in fact refers to a number of diverse cities, populations is quite appropriate IMO.
For reference, pl look at this site, <http://www.wordhippo.com/what-is/sentences-with-the-word/populations.html.>
In any case, if
...
For reference, pl look at this site, <http://www.wordhippo.com/what-is/sentences-with-the-word/populations.html.>
In any case, if
...
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