| No | Pattern | Freq | Relations |
|---|---|---|---|
| 1 | [prep] a prolonged period [prep] | 20 |
|
| 2 | [prep] a prolonged period [prep] | 20 |
|
| 3 | [prep] a prolonged period of | 18 |
|
| 4 | [prep] a prolonged period of | 18 |
|
| 5 | over a prolonged period [prep] | 9 |
|
