Which of the following statements about aggregate planning strategies that managers may use in the face of fluctuating demand is true? a. Short-term changes in facilities and equipment are often used in traditional aggregate planning methods because of the low capital costs involved. b. In firms that consist primarily of jobs with low skill requirements, changing workforce levels is not a cost-effective option. c. Carrying back orders or tolerating lost sales during peak demand periods may be unacceptable if profit margins are high and competition is low. d. Subcontracting during periods of peak demand is effective in industries that manufacture a large portion of their own parts