Today is 1 July, 2019. Siobhn has a portfolio which consists of two different types of financial instruments (henceforth referred to as instrument A and instrument B). Siobhn purchased all instruments on 1 July 2010 to create this portfolio, which is composed of 22 units of instrument A and 26 units of instrument B. - Instrument A is a zero-coupon bond with a face value of $100. This bond matures at par. Its maturity date is 1 January 2029. - Instrument B is a Treasury bond with a coupon rate of j2=4.32% p.a. and a face value of $100. This bond matures at par. Its maturity date is 1 January 2022. Calculate the current duration of Siobhn's portfolio using a yield to maturity of j2=4.33% p.a. Express your answer in terms of years and round your answer to two decimal places. a. 4.61 years b. 6.06 years c. 6.72 years d. 4.96 years