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Which of the following statements is correct? Select one: O a. economic variables measured in physical units are actual variables, and economic variables measured in monetary units are nominal variables O b. economic variables measured in physical units are real variables, and economic variables measured in monetary units are nominal variables O c. economic variables measured in physical units are nominal variables, and economic variables measured in monetary units are real variables O d. economic variables measured in physical units are real variables, and economic variables measured in monetary units are actual variables