Seal Island

Hello Sir,
Thanks for your earlier answer to my query sir. I have asked this question before but I am stuck again its a-iii, sensitivity analysis and measuring changes in decommissioning costs before npv becomes zero why do we have to calculate the real discount rate for it and why do we apply GDP growth of 4% in the formula instead of inflation rate of 2%. Do you have any lecture on this particular topic which you can refer,please . Thank You