Heidi Company is considering the acquisition of a machine that costs $655,200. The machine is expected to have a useful life of six years, a negligible residual value, an annual net cash flow of $117,000, and annual operating income of $78,000. What is the estimated cash payback period for the machine (round to one decimal point)? a. 5.6 years b. 1.5 years c. 8.4 years d. 3.4 years