A mutual fund is holding a portfolio consists of four bonds. Duration of Bond A is 1.22-year, of Bond B is 2.75-year, of Bond C is 3.11-year, of Bond D is 5.16. The proportion of Bond A in the portfolio is 30%, Bond B is 25%, Bond C is 15%, and the rest goes to Bond D.

a) Which bond in the portfolio is exposed to the lowest interest rate risk? Briefly explain.

b) What is the duration of the portfolio?