Hello can you please help with this?

Green Chemical Inc has a liability due in 15 years whose present value is $1,000 and the current interest rates are 7.5%. Firm intend to immunize this obligation by purchasing $1,000 worth of bonds or a combination of bonds Firm has the following three bonds: Bond 1 has 15 years remaining until maturity, a coupon rate of 8.00% and a face value of $1,000 Bond 2 has 20 years until maturity, a coupon rate of 8.5%, and a face value of $1,000 Bond 3 has 35 years until maturity, a coupon rate of 5.5% and a face value of $1,000 A. Which bond should Green Chemical Inc prefer to immunize their liability? B. Also, if you have to suggest a combination of two bonds, which two bonds will you suggest and in what combination