A student investigates how stretching a spring with different forces affects its extension. The results are shown in the table below.
| Force (N) | Extension (cm) |
|---|---|
| 0.5 | 1.5 |
| 1.0 | 3.0 |
| 1.5 | 4.5 |
| 2.0 | 6.0 |
| 2.5 | 7.5 |
| 3.0 | 9.0 |
The student writes this conclusion:
"The extension of the spring is directly proportional to the force applied."
Comment on the student's conclusion.