What is the payback period of a project with the following expected cash inflows (in millions) as shown below? Suppose that the project requires an upfront investment of $250 million and no other costs in all other years Year 1-$10 Year 2 $10 Year 3-$10 Year 4-$90 Year 5-$90 Year 6-$90 Year 7-$100 O 7 years O 6 years 8 years O 5 years