A business signs a $255,000, 4-year promissory note with its supplier when it purchased equipment. The full loan amount with accrued interest is paid on the note's maturity. A single payment of $331,140 is made at the end of year 4. What was the loan's interest rate? Select one: a. 6.67% b. 6.82% c. 6.75% d. 6.59% e. 6.63%