The town of Bookville has a massive public library system. Bookville's municipal government faces a trade off between retaining the current number of library workers and replacing some of its workers with computers. The four options available to the government are given in table below.

	Number of workers	Number of computers
Choice A	30	50
Choice B	20	100
Choice C	10	150
Choice D	5	300

Computers and software cost $5,000 each and must be replaced each year. In a non-unionized labor market, workers are paid $20,000 per year. In these circumstances, the optimal choice from a cost perspective for the city government is