[顯然][是]¶
Without testing this collocation with paradigmatic alternants of different word classes in the slot, Huang–Shih §12.2 jump to the conclusion that 顯然是 is an adverb. Compare, however:
| with 是 as | |
|---|---|
| 選舉有必要 | Ø |
| 選舉顯然有必要 | Ø |
| 選舉'是有必要 | adverb |
| *選舉顯然是有必要 | copula |
| 選舉是有必要的 | copula |
| 選舉顯然是有必要的 | copula |