求问B选项的原理是啥，没太懂

问题如下：

Which of the following statements is correct?

选项：

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.

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.

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.