Which of the following statements is true? a. by investing in varied and numerous assets, an investor is able to virtually eliminate all asset-specific risks in her portfolio, both easily and cheaply. b. it is possible, but not very easy, for an investor to control market-wide risks in his portfolio, and increases in these market-wide risks are costly because they reduce expected returns. c. the most important characteristic in determining the expected return of a well-diversified portfolio is the total variance risks of the individual assets in the portfolio. d. when a portfolio has a positive investment in every one of its assets, its standard deviation cannot be less than that on every asset in the portfolio.