ravel-unravel-gravel-dredge-engrave-grave

ravel
He was trying to ravel out the complicated series of events that had led to this situation.
他那时正试图把导致这一局面的一系列复杂事件理出个头绪来。
I unravelled the string and wound it into a ball.
我把绳子解开并绕成一个球。

The discovery will help scientists unravel the mystery of the Ice Age.
这一发现将有助于科学家揭开冰川时代的奥秘。  
gravel
The car drew up with a crunch of gravel.
那辆汽车在沙砾路上嘎吱一声停了下来。
They dredge the bay for gravel.
他们在挖掘海湾沙砾。
Police have spent weeks dredging the lake but have not found his body.
警方已经花了数周对湖底清淤,但还是没有发现他的尸体。
grave
They stood in silent homage around the grave.
他们恭敬地站在坟墓周围致默哀礼。

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engrave
It is difficult to engrave in marble.
大理石很难雕刻。