. . . "fied unit and pays its proportional amount of such unit is total costs, pursuant to a contract: (a) For the life of the unit; (b) For a cumulative term of no less than 30 years, including contracts that permit an election for early termination; or (c) For a period no less than 25 years or 70 percent of the economic useful life of the unit determined as of the time the unit is built, with option rig" .