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: a. SPI = Total activity cost for supplier / Total purchase cost with that supplier to. SPI = Total activity cost for that supplier / Total cost of ownership for that supplier c. SPI = Total purchase cost for that supplier] Total Activity Costs for that supplier\" d. SPI = Total purchase cost / Total cost of doing business