| No | Pattern | Freq | Relations |
|---|---|---|---|
| 1 | a period of [prep] | 42 |
|
| 2 | a period of [v-ing] | 32 |
|
| 3 | period of [v-ing] [prep] | 27 |
|
| 4 | a period [v-ing] [prep] | 13 |
|
