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