Which One of the Following Is a Correct Statement Concerning

Which one of the following is a correct statement concerning the weighted average cost of capital?

A) Preferred stock is generally the cheapest form of external financing.
B) An increase in interest rates by the Bank of Canada generally has no effect on a firm's cost of capital.
C) The use of debt generally lowers a firm's cost of capital, at least initially.
D) Taxes help lower the cost of debt and preferred stock financing.
E) The weighted average cost of capital remains constant for a firm over time.