他正努力寻找破案的关键线索。

He is trying hard to find the key ___clue to solve the case.

这个指纹是破案的重要线索。

The fingerprint is a crucial ___clue to solving the case.

这个奇怪的脚印是破案的重要线索。

The strange footprint was an important ___clue to solve the case.

这个填字游戏的线索太难了，我想不出答案。

The ___clue in this crossword puzzle is so difficult that I can't figure out the answer.