| No | Pattern | Freq | Relations |
|---|---|---|---|
| 1 | [noun] have been a key [noun sg] | 18 |
|
| 2 | [noun] has [been] a key [noun sg] | 14 |
|
| 3 | [noun sg] have [been] a key [noun sg] | 12 |
|
| 4 | [noun] have [been] a key factor | 6 |
|
