For a firm considering expansion of its existing line of business, why is the WACC, rather than

Question:

For a firm considering expansion of its existing line of business, why is the WACC, rather than the cost of equity, the preferred if the firm has both debt and equity in its capital structure?
Discount Rate
Depending upon the context, the discount rate has two different definitions and usages. First, the discount rate refers to the interest rate charged to the commercial banks and other financial institutions for the loans they take from the Federal...