Which of the following statements is true? O a. Beta is the slope of the security market line. O b. A stock with a beta of 1.00 has a required return equal to the required return on the market. O c. Adding a stock with a high standard deviation to a portfolio always increases the risk of the portfolio. O d. The higher is a stock's beta, the lower is the standard deviation of the stock's returns. O e. both b and d are correct