HW help. The X's are the ones i already tried.

Which of the following portfolio is risk free? Question 7 Incorrect Mark 0 out of 5 Remove flag Select one: Xa. A portfolio consisting of $300,000 worth of long-term U.S. T-bonds. b. A portfolio consisting of $300,000 worth of U.S. corporate junk bonds. X 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 US TIPS