Definition: a situation which is very difficult to understand Level: C2
他突然辞职的真正动机仍是个谜。
The true motive behind his sudden resignation remains a ___puzzle.
考古学家仍在拼凑这个古老文明崩溃的谜团。
Archaeologists are still piecing together the ___puzzle of the ancient civilization's collapse.
相互矛盾的证据给调查人员出了一个真正的难题。
The inconsistent evidence presented a real ___puzzle for the investigators.
