In a Peabody Award winning program, NPR correspondents argued that a "Giant Pool of Money" (represented by $70 trillion in worldwide fixed income investments) sought higher yields than those offered by U.S. Treasury bonds early in the decade. This pool of money had roughly doubled in size from 2000 to 2007, yet the supply of relatively safe, income generating investments had not grown as fast. Investment banks on Wall Street answered this demand with products such as the mortgage-backed security and the collateralized debt obligation that were assigned safe ratings by the credit rating agencies.

What is one investment assigned safe ratings by the credit rating agencies?