CharCore mixes together wood chips and pine oil. After joint manufacturing costs of $2,000 have been incurred, the mixture separates into two products, granulated charcoal and methyl alcohol. At the split-off point, granulated charcoal can be sold for $5,000 and the alcohol can be sold for $9,000. The charcoal can be further processed at a cost of $6,000 to make air filters which could be sold for $15,000. The alcohol can be further processed at a cost of $7,000 to make a cleaning solvent which could be sold for $14,000. What is the net increase (decrease) in operating income from air filters?
A. $3,000
B. $2,000
C. $(2,000)
D. $4,000