Galdrian must choose the discount rate for a restoration provision whose cash flows are already adjusted for all the risks specific to the obligation. It has four rates: 3.1% (post-tax risk-free rate), 6% (pre-tax rate including a premium for the obligation's risks), 4.2% (pre-tax rate reflecting only the current time value of money) and 2.5% (rate used when the provision was set up six years ago). Which rate should be used?