You plan to go to graduate school for 2 years beginning in year 6. Tuition is $28,319 per year, due at the end of each school year. To pay for future tuitions, construct a portfolio that is immune to interest rate risk by buying a single issue of a zero-coupon bond. What maturity zero should you buy? Assume a flat yield curve of 0.04.

Assume annual compounding. In the above description, if you see a flat yield curve of 0.08 for example, then it means that the yield at all maturities is 8%.