The economic life of a machine is Select one: a. the number of years before the first major repair is required. O b the optimal number of years that the machine should be kept from the point of view of maximizing the present worth of the company's cash flows. c. the number of years that the corresponding project is intended to last if the project is finished before the machine is completely used up the machine can be sold and the after-tax salvage value included in the project's cash flows d. the number of years that the machine is capable of performing its intended function Assume that an existing machine will be replaced with a challenger and that the company needs to decide when is the best time for the replacement. The marginal cost of keeping the defender for another year is 589,000, the marginal cost of keeping the defender for another year afterwards (for the second year) is $93,000, the marginal cost of keeping the defender for the 3rd year is $95.000, and the marginal cost of keeping the defender for the 4th year is 596,000. The challenger's equivalent uniform annual cost is $92,000 if the challenger is used for 1 year, 591,000 if the challenger is used for 2 years, 594,000 if the challenger is used for 3 years, and $94,000 if the challenger is used for 4 years. When should the defender be replaced? Select one: a immediately b. The defender should be kept for 4 more years and then replaced c The defender should be kept for 1 more year and then replaced. d. The defender should be kept for 3 more years and then replaced. e. The defender should be kept for 2 more years and then replaced