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,000th unit produced and sold will be about 90 percent of the cost of the 500th unit, and the 2,000th unit will be 90 percent of the l,000th unit. Therefore, if the cost of the 500th unit is $100, the cost of the 4,000th unit would be

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