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Answer (C) is correct . A change in market share reflects a change in relative competitiveness. To isolate the effect on operating income of an increase or a decrease in market share, the company must know its budgeted and actual market shares, the actual size of the market for November, and the budgeted weighted-average unit contribution margin. Such computations may help Folsom to determine whether its decline in sales resulted from a loss of competitiveness or a shrinkage of the market.

Answer (A) is incorrect because Folsom will need to know the actual total market size.
Answer (B) is incorrect because Folsom will need to know the actual total market size.
Answer (D) is incorrect because Folsom will need to know the budgeted market share.