A bond investor has the choice of investing in one of four 15-year bonds. Bond A pays an annual coupon of 3.5%, Bond B pays an annual coupon of 3.1%, Bond C pays an annual coupon of 4.0%, and Bond D pays an annual coupon of 4.3%. Holding all else constant, if the bond investor is extremely worried about interest rate risk and would like to buy a bond with the lowest exposure, which bond should he buy?