Explanation:

- First year: Typically, the base year of an economic index is selected as the first year in a series. Analysts often choose this year because it serves as a point of comparison for all subsequent data, making it a reference point against which changes are measured.

- Last year: Not commonly chosen as the base, because using the last year would make constant updating necessary and complicate comparisons over time.

- Twentieth year: There's no standard practice for selecting the twentieth year unless it offers specific advantages, such as stability in the economy.

- Tenth year: Occasionally chosen for convenience, but it's less common for maintaining consistency in long-time comparisons.

The correct Answer is: First year