Answer:
Investors are risk averse, which means that they are willing to invest in low risk projects or investments. In order for an investor to invest in a riskier project, he/she will expect to receive higher returns to compensate for the extra risk. US Treasury bonds are probably the safest investments in the world, that is why they yield the lowest interest rate. AAA bonds are less risky than BBB bonds, which in turn are less risky than CCC bonds. That is why AAA bonds yield a lower return than BBB bonds, and BBB bonds yield a lower return than CCC bonds.
