Frostire industries is considering issuing one-year debt. The estimates of the value of the interest tax shield and the probability of distress for different levels of debt are as follows. Suppose in the event of distress, the present value of distress costs is equal to $20 million. The optimal level of debt for Frostfire Industries is: a $50 milition b. $70 million c. $40 million d. $60 milison