Consider a regional economy with 12 million workers and an urban utility curve u(n) = 15 + 12n - 2n^2, where n is the number of workers in millions. Initially the region has four cities (A, B, C, D), each with a population of 3 million. Suppose city A introduce carplanes to replace regular cars for commuting, and its urban utility function is u(n) = 15 + 12n -n^2. Graphically illustrate and discuss the effect of the carplane on (i) equilibrium distribution of population between the cities and (ii) regional utility. Please show the graphs as well!!