Silvie would like to send her twin children to university. She would like to accumulate $60,000 by he time her children are ready for university in 15 years. She is a conservative investor and prefers a passive investment approach. Which of the following bond strategies would you recommend to Silvie in order to eliminate interest rate risk? a) purchase three strip bonds with maturities of 5,10 and 15 years respectively b) a laddered approach c) a swapping approach d) an immunization approach