| No | Pattern | Freq | Relations |
|---|---|---|---|
| 1 | [vb past] [prep] be introduced | 24 |
|
| 2 | [vb past] [prep] be introduced | 23 |
|
| 3 | [vb past] to [be] introduced | 23 |
|
| 4 | [vb past] [prep] [be] introduced | 23 |
|
| 5 | had [prep] [be] introduced | 11 |
|
