A project has a Budget at Completion (BAC) of $500,000, an Actual Cost (AC) of $200,000, and an Earned Value (EV) of $180,000. What is the project's Cost Performance Index (CPI), and what does it indicate about the project's cost efficiency?
1.25, indicating the project is under budget
1.0, indicating the project is on budget
0.90, indicating the project is over budget
0.80, indicating the project is on track