A presently owned machine can last 3 more years if properly maintained at a cost of $16,000 per year. Its A0C is $25,000 per year. After 3 years, it can be sold for an estimated $7,000. A replacement costs $86,000 with a $10,000 salvage value after 3 years and an operating cost of $21,000 per year. Different vendors have offered $10,000 and $28,000, respectively, for the current system as a trade-in for the replacement machine. At i=13% per year, perform a replacement study and determine whether the defender be retained or replaced. According to the replacement study, \begin{tabular}{l} (Click to select) v \\ \hline (Click to select) \\ retain defender \\ select challenger \end{tabular}