A portfolio manager utilizes a valuation model to assess the worth of a bond portfolio currently valued at USD 125.00 million, assuming a flat term structure of interest rates. According to the model, if all interest rates decrease by 20 basis points (0.20%), the portfolio's value is expected to increase to USD 127.70 million. Conversely, if all interest rates increase by 20 basis points, the portfolio's value is projected to decrease to USD 122.20 million. Given these projections, determine which of the following options is closest to the effective duration of the bond portfolio.