The concept of compound interest refers to a. the process of gradually retiring a debt through periodic payments of principal and interest. b. the process of servicing a debt with regular interest payments, followed by a lump sum payment of principal and interest at the end of the loan term. c. the process of converting future lump sums and annuities into present values at a stated interest rate. d. the process of earning interest on an original amount, plus interest on interest previously earned.