Which of the following statements is correct regarding empirical tests of the CAPM? A. The U.S. stock market represents an exhaustive set of marketable securities which is necessary to define the market portfolio in CAPM. B. A firm's beta can be accurately estimated using historical data. C. The ideal proxies for the market risk premium and CAPM betas are backward looking. D. Predictive regressions can be used to obtain a more accurate proxy of the conditional market risk premium compared to the historical average market excess return