科学家多年来一直试图解决这个难题。

Scientists have been trying to solve this ___puzzle for years.

生命的意义是哲学家们深思的一个深刻谜题。

The meaning of life is a profound ___puzzle that philosophers ponder.

相互矛盾的证据使这个案件完全成了一个谜。

The conflicting evidence made the case a complete ___puzzle.