Thunderhorse Oil. Thunderhorse Oil is a U.S oil company. Its current cost of debt is 7.00%, and the 10 -year U.S. Treasury yield, the proxy for the risk-free rate of interest, is 3.00%. The expected return on the market portfolio is 8.00%. The company's effective tax rate is 39%. Its optimal capital structure is 60% debt and 40% equity. a. If Thunderhorse's beta is estimated at 1.10 , what is Thunderhorse's weighted average cost of capital? b. If Thunderhorse's beta is estimated at 0.80 , significantly lower because of the continuing profit prospects in the global energy sector, what is Thunderhorse's weighted average cost of capital