The assumption that the firm's debt-to-equity ratio is constant implies that: the firm's risk of debt and equity will change when the firm undertakes a new project. the new project has average risk. the firm's cost of capital will not change when it invests in a new project. O corporate taxes are the only imperfection. None of the above Of the inputs required in the Black-Scholes formula, which one is not directly observable? The current stock price The exercise price of the option The risk-free interest rate The number of days to the expiration of the option The volatility of the stock price