Rank the following bonds in order from lowest credit risk to highest risk, all with the same

Question:

Rank the following bonds in order from lowest credit risk to highest risk, all with the same time to maturity, by their yield to maturity: Treasury bond with yield of 4.65 percent, United Airline bond with yield of 9.07 percent, Bank of America bond with a yield of 6.25 percent, and Hewlett/Packard bond with yield of 6.78 percent.