| No | Pattern | Freq | Relations |
|---|---|---|---|
| 1 | [prep] a fixed period [prep] | 29 |
|
| 2 | [prep] a fixed period of | 25 |
|
| 3 | for a fixed period [prep] | 18 |
|
| 4 | for a fixed period [prep] | 18 |
|
