Nighthawk inc. is considering disposing of an old machine with a book value of $22,500 and an estimated remaining life of three years. the old machine can be sold for $6,250. a new machine with a purchase price of $68,750 is being considered as a replacement. it will have a useful life of three years and no residual value. it is estimated that the annual variable manufacturing costs will be reduced from $43,750 to $20,000 if the new machine is purchased. the differential effect on income for the entire three years for the new machine is a(n) a. $8,750 decrease b. $8,750 increase c. $2,925 decrease d. $31,250 decrease