问题如下：
Matt, CFA, is investigating the capital structure of Company M, a manufacturer of automobile. The company is in the 25% tax bracket. Matt expects that a change in financial leverage may result in different WACC. He has estimated different situations in the following table. Under which of the following situations, Company M`s WACC is smallest?
选项：
A. E/A = 75%
B. E/A = 70%
C. E/A = 60%
解释：
B is correct.
考点：Static Trade-Off Theory
解析：根据公式
当D/A = 25%, E/A = 75%，代入得WACC = 0.25 * 7% * (1-25%) + 0.75 * 10% = 8.8125%
当D/A = 30%, E/A = 70%，代入得WACC = 0.3 * 7.2% * (1-25%) + 0.7 * 10.25% = 8.795%
当D/A = 40%, E/A = 60%，代入得WACC = 0.4 * 8.5% * (1-25%) + 0.6 * 11.5% = 9.45%
total capital 是E吧，为什么答案把debt to total capital等同于E/A
