Which of the following portfolio is risk free? Select one: O a. A portfolio consisting of $300,000 worth of long-term U.S. T-bonds. O b. A portfolio consisting of $300,000 worth of AAA-rated U.S. corporate 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. O d. A portfolio consisting of $300,000 worth of U.S. corporate junk bonds. o e. A portfolio consisting of $300,000 worth of U.S. TIPS