The following table shows the annual effective interest rates being credited by an investment account, by calendar year of investment. The investment year method is applicable for the first 3 years, after which a portfolio rate is used: Investment Year Rates iz Calendar Year of Investment 1990 1991 1992 1993 i 10% iz 10% Calendar Year of Portfolio Rate 1993 1994 Portfolio Rate 8% (1 19% 12% 5% 10% 8% (t - 2)% 12% 1995 6% 9% 11% 6% 1996 9% 1994 7% 7% 10% 1997 10% An investment of 100 is made at the beginning of years 1990, 1991, and 1992. The total amount of interest credited by the fund during the year 1993 is equal to 28.40. Calculate t. O a. 7.75 O b. 7.25 O c. 7.00 O d. 8.00 e. 7.50