Section 4(a) of the NGA of 1938 specifies that all rates and charges made, demanded, or received by any natural gas company for or in connection with the transportation or sale of natural gas ... shall be "just and reasonable." The term "just and reasonable" has been interpreted to apply to rates which are assessed without undue bias or discrimination that allow the natural gas companies to recover all prudently incurred costs applicable to rate payers, including a return on capital (i.e., the cost of capital) sufficient to maintain and support its credit and enable it to raise the money necessary for the proper discharge of its public duties.