问题如下：
Which of the following statements is correct?
选项：
A. The appropriate tax rate to use in the adjustment of the before-tax cost of debt to determine the after-tax cost of debt is the average tax rate because interest is deductible against the company's entire taxable income.
B. For a given company, the after-tax cost of debt is generally less than both the cost of preferred equity and the cost of common equity.
C. For a given company, the investment opportunity schedule is upward sloping because as a company invests more in capital projects, the returns from investing increase.
解释：
B is correct.
Debt is generally less costly than preferred or common stock. The cost of debt is further reduced if interest expense is tax deductible.
老师 请问marginal tax rate 和 average tax rate有什么区别呢？什么是capital project呢？