Consider following expected costs of using current machine (defender): a. Fill in four blank cells above (PW and AE of Defender and Challenger if they are sold in yr. 1, and if they are sold in yr. 2) b. Economic service life of the defender is ___ years c. Economic service life of the challenger is ___ years d. Defender ___ be replaced by challenger. e. ff defender must be replaced by challenger, it should be done ___. Show work or explain to get credit for part (c) Now/At the end of year 1/at the end of year 2