Using data from FRED, plot the series "Moody's Seasoned Aaa Corporate Bond Yield", representing the borrowing cost for companies with high credit ratings, and "Moody's Seasoned Baa Corporate Bond Yield", representing the borrowing cost of companies with poor ratings. Plot the credit spread, i.e. the difference between these two series, and discuss its evolution during this period. Explain the evolution of the corporate bonds market during this period based on these plots and the discussion in the specialized press