The trend toward higher population and population density in urban areas like Chicago is best explained by A) a combination of foreign immigration and domestic migration from rural areas.
B) government subsidies that encouraged migration to urban centers.
C) better educational opportunities in urban centers, which drew rural families.
D) lower rates of taxation in urban areas, which resulted in higher standards of living.

lower rates of taxation in urban areas, which resulted in higher standards of living.

the correct answer is:   "   3.6 % " .

          →   the unemployment rate is:   " 3.6 % " .

explanation:

to calculate the "unemployment rate" :

the "unemployment rate"   =

  [# of people unemployed] /   [working age population]   * 100 %   ;

                =   [9 million] / [250 million]     *   100 % ;

    →     note:   the units of "million" cancel out;

                    since:   " {million} / {million} = 1 " ;

                =   (9 / 250)   * 100 %   ;

                = 0.036 * 100 % ;

                =   3.6 %

answer:   the unemployment rate is:   " 3.6 % " .