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A is corrent. The requirement is to calculate the amount by which income will increase if the tables line is discontinued. To make the determination you would compare the amount of revenue from renting the factory space to the contribution margin less the avoidable costs from producing tables. The unavoidable costs allocated to the tables line would be incurred regardless of the decision and therefore are irrelevant. A is corrent because income would increase by $18,000 [$24,000 rent — ($18,000 contribution margin — $12,000 avoidable costs].
B is incorrect. Income would increase by $18,000. 
C is incorrect. Income would increase by $18,000. 
D is incorrect. Income would increase by $18,000.