Consider a problem in which roads are to be built that connect all four cities a, b, c, and d to one another. In other words, after the roads are built, it will be possible to drive from any one city to another. The cost (in millions) of building a road between any two cities is provided in the following: Cities a B c D
a 0 30 20 50
b 30 0 50 10
c 20 50 0 75
d 50 10 75 0
Using this table, find a set of roads of minimum cost that will connect the cities.