Suppose that a city's energy demand is 30 megawatts during off-peak times, and 40 megawatts at its peak. The city has been purchasing electricity from an outside company but has decided to build its own power plants to satisfy all of its energy demand. The city can choose to build one or more plants to generate the needed electricity. There are three types of plant: coal, natural gas, and hydroelectric. The three types of plants face the costs appearing in the table above. Assuming the city's power needs will not change in the foreseeable future, to achieve the lowest cost of power generation the city should build?