If a Firm Estimates That Its Costs Will Fall by 10

If a firm estimates that its costs will fall by 10 percent each time volume doubles for its product, then the cost of the l,000^th unit produced and sold will be about 90 percent of the cost of the 500^th unit, and the 2,000^th unit will be 90 percent of the l,000^th unit. Therefore, if the cost of the 500^th unit is $100, the cost of the 4,000^th unit would be

A) $0.
B) $72.90.
C) $81.00.
D) $90.00.
E) $100.00.