Which statement is FALSE regarding WACC and its components?

Which statement is FALSE regarding WACC and its components?

a)The cost of debt is usually less than the cost of equity.

b)The WACC should be used as the discount rate for all projects that the firm considers.

c)For an all-equity firm, the cost of equity equals the WACC.

d)The WACC may increase if the firm seeks external financing for a project.

Please explain why b) is the correct answer.