(Practice using perturbation method to estimate duration in this quetion) Given that interest rate is 5%. Consider a mortgage that produces $10000 a year cashflow and matures in 20 years. Step 1: calculate the price of the mortgage in excel Step 2: change the interest rate to 4.99% and 5.01%, and calculate the respective price of the mortgage in those cases Step 3: Calculate the duration of the mortgage with the formula in "Duration: Estimate" What is the duration of the mortgage? O 7.90 O O 8.90 O 9.90 O 10.90