The town of Bookville has a massive public library system. Bookville's municipal government faces a tradeoff between retaining its current level of library workers and replacing some of them 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. After the introduction of a strong union, the annual salary for workers increases to $30,000. In these circumstances, the optimal choice from a cost perspective for the city government is...?