City Gas is a natural monopoly that supplies natural gas to a particular city. Its cost and demand information are given below.

Quantity (Millions of therms)	Price ($ per therm)	Total Cost (million $)
1	48	35
2	44	64
3	38	90
4	30	113
5	20	133
6	8	150

The marginal cost of going from a production of 4 million therms to a production of 5 million therms is

		$133 million
		$113 million
		$23 million
		$20 million