Which of the following statements is correct? a. A stock's beta is less relevant as a measure of risk to an investor with a well-diversified portfolio than to an investor who holds only one stock. b. A security's beta measures its diversifiable (firm-specific) risk relative to that of other securities. c. A stock's beta can be calculated by comparing its returns to the market's returns over some time period because the beta coefficient measures a stock's volatility relative to market. d. If the returns of two firms are negatively correlated, one of them must have a negative beta. e. Combining stocks that are perfectly negatively correlated and that have the same beta coefficient into a portfolio is riskier than holding an individual stock, because the portfolio will not benefit from diversification.