Suppose There Are Three Power-Generating Plants, Each of Which Has

Multiple Choice

Suppose there are three power-generating plants, each of which has access to 5 different production processes. The table below summarizes the cost of each production process and the corresponding number of tons of smoke emitted each.  
The least costly way of lowering smoke emissions from 12 tons to 9 tons per day would be for:

A) each firm to reduce emissions by 1 ton, emitting 3 tons each.
B) Firm X to emit 2 tons, Firm Y to emit 3 tons and Firm Z to emit 4 tons.
C) Firm X to emit 4 tons, Firm Y to emit 3 tons and Firm Z to emit 2 tons.
D) Firm X to emit 1 ton, Firm Y to emit 4 tons and Firm Z to emit 4 tons.