A business is evaluating their suppliers and has decided to use the 'Supplier Performance Index' (SPI) method. Which of the following is the correct formula for the SPI? Select one: 0 a. SPI = Total purchase cost I Total cost of doing business 0 b. SPI = Total activity cost for supplier 1' Total purchase cost with that supplier O c. SPI = Total purchase cost for that supplier] Total Activity Costs for that supplier 0 d, SPI = Total activity cost for that supplier / Total cost of ownership for that supplier