| No | Pattern | Freq | Relations |
|---|---|---|---|
| 1 | is [prep] three years ' duration | 24 |
|
| 2 | be [prep] three years ' duration | 24 |
|
| 3 | [be] [prep] three years ' duration | 24 |
|
| 4 | [is] [prep] three years ' duration | 24 |
|
