A table of the US minimum wage for 6 different years is shown below.
| Year | US minimum wage (dollars per hour) |
|---|---|
| 1960 | 1.00 |
| 1970 | 1.60 |
| 1980 | 3.10 |
| 1990 | 3.80 |
| 2000 | 5.15 |
| 2010 | 7.25 |
In which 10-year period shown in the table was the percent increase in the US minimum wage the greatest?
1970 to 1980
1980 to 1990
1990 to 2000
2000 to 2010
