Assume a flat yield curve environment of 10% yields, along with the following bonds and their durations: Bonds A, B, and C has durations of 5, 10, and 15 years, respectively. Further assume that the three bonds are of equal value and are the only bonds in existence. Set up a single-index representation of their covariance. What is the covariance between all pairs of bonds? (Hint: Use the properties of Sharpes single index model.)