(b) In the data file capm4.xlsx (http://www. principlesofeconometrics . com/poe4/poe4excel. htm) are data on the monthly returns of six firms (Microsoft, GE, GM, IBM, Disney, and Mobil-Exxon), the rate of return on the market portfolio (MKT ), and the rate of return on the risk free asset (RISKFREE). The 132 observations cover January 1998 to December 2008. Estimate the CAPM model for each firm, and comment on their estimated beta values. Which firm appears most aggressive? Which firm appears most defensive? (c) Finance theory says that the intercept parameter a; should be zero. Does this seem correct given your estimates? For the Microsoft stock, plot the fitted regression line along with the data scatter. (d) Estimate the model for each firm under the assumption that a; = 0. Do the estimates of the beta values change much