2. In the world of CAPM, market portfolio M is the optimal risky asset portfolio. In the U.S. market, the market index represented by the Standard & Poor's has been hitting new highs for a long time. In the Chinese A-share market, the market index represented by the Shanghai Composite Index has often hit new lows. Based on your observations and thinking, talk about the similarities and differences between the two indices. Why is the market portfolio of investing in U.S. stocks after the 2008 financial tsunami in the United States different from investing in Chinese stock market portfolios and in different directions?