Assume in analyzing alternative proposals that Proposal F has a useful life of 6 years and Proposal 3 has a useful life of 9 years. What is one widely used method to make the net present values of the proposals comparable? a. Ignore the useful lives of 6 and 9 years and find an average (71/2 years). b. Ignore the fact that Proposal F has a useful life of 6 years and treat it as if it has a useful life of 9 years. c. Adjust the life of Proposal J to a time period that is equal to that of Proposal F by estimating a residual value at the end of Year 6. d. Ignore the useful lives of 6 and 9 years and compute the average rate of retur