help with finance question

Suppose you owned a portfolio consisting of $250,000 of U.S. government bonds with a maturity date of 30 years. Would your portfolio be riskless? What if your portfolio consisted of $250,000 of 30-day Treasury bills? Every 30 days your bills mature, and you reinvest the principle ($250,000) in a new batch of bills. Assume that you live on the investment income from your portfolio and that you want to maintain a constant standard of living. Is your portfolio truly riskless? Can you think of any asset that would be completely riskless? What security comes closest to being riskless? Explain.

I am using financial management by brigham and ehrhardt