Heidi Company is considering the acquisition of a machine that costs $597,000. The machine is expected to have a useful life of 6 years, a negligible residual value, an annual net cash inflow of $129,000, and annual operating income of $88,309. The estimated cash payback period for the machine is (round to one decimal point) a. 4.6 years b. 7.5 years c. 6.0 years d. 6.8 years