Study the table below, which shows population data for Lagos, Nigeria.
| Year | Total population (millions) | Annual growth rate (%) | Growth due to migration (%) | Growth due to natural increase (%) |
|---|---|---|---|---|
| 1991 | 5.0 | 5.0 | 60 | 40 |
| 2001 | 8.0 | 4.5 | 55 | 45 |
| 2011 | 12.5 | 4.0 | 48 | 52 |
| 2021 | 15.0 | 3.0 | 46 | 54 |
Identify the year in which the absolute annual population increase due to natural increase was the greatest.
1991
2001
2011
2021