Which of the following portfolio is risk-free?

A) A portfolio consisting of $300,000 worth of US TIPS.

B) A portfolio consisting of $300,000 worth of long-term U.S. T-bonds.

C) A portfolio consisting of $300,000 worth of 30-day T-bills. Every 30 days the T-bill matures, and you reinvest the principal in a new batch of T-bills.

D) A portfolio consisting of $300,000 worth of AAA-rated U.S. corporate bonds.

E) A portfolio consisting of $300,000 worth of U.S. corporate junk bonds.