Look back to your answer to Self-Test 18.1. Serial has accumulated $40 million of surplus cash, which is earning a 1.5% after-tax interest rate. It can use the cash to pay for a new synthesizer line. The line has the same business risk as Serial’s other assets. Should the future cash flows from the new line be discounted at the 1.5% interest rate or at Serial’s after-tax WACC? Explain briefly.