Airbnb used a 25-year, $50 million bond issue to finance its expansion. In its plan to ensure that funds would be available to redeem the bonds at maturity, it arranged that none of the bonds would mature during the first 15 years. Thereafter, 10 percent of the bonds mature each year until all the bonds are retired at the end of the 25th year. Which method of bond repayment is Airbnb using? O Registered bond O Selling old bonds Sinking fund Selling new bonds Serial bond