According to the table below, a cottonwood tree and a black walnut tree each currently have a diameter of 18 inches. What is the closest difference, in inches, of their diameters 20 years from now?
| Species of tree | Growth factor |
|---|---|
| Red maple | 4.5 |
| River birch | 3.5 |
| Cottonwood | 2.0 |
| Black walnut | 4.5 |
| White birch | 5.0 |
| American elm | 4.0 |
| Pin oak | 3.0 |
| Shagbark hickory | 7.5 |
5.4
5.5
5.6
5.7
