Which of the Following Statements Regarding Calculating a Firm's Cost

Which of the following statements regarding calculating a firm's cost of capital is correct?

A) The after-tax cost of debt is generally more expensive than the after-tax cost of preference share.
B) Since retained earnings are readily available, the cost of retained earnings is generally lower than the cost of debt.
C) If a company's beta increases, this will increase the cost of capital.
D) The level of general economic conditions will determine whether a firm should utilize an arithmetic average cost of capital or a weighted average cost of capital.