All other risk being equal, which of the following bonds would have the highest interest rate risk? \begin{tabular}{|l|l|} \hline a & a low coupon bond of 100 years to maturity \\ \hline b & a high coupon bond of 100 years to maturity \\ \hline c & a low coupon bond of 10 years to maturity \\ \hline d & a high coupon bond of 10 years to maturity \\ \hline e & none of the above \end{tabular}