It sounds all wrong--drilling holes in a piece of wood to make it more resistant to knocks.But it works because the energy from the blow gets distributed throughout the wood rather than focusing on one weak spot.The discovery should lead to more effective and lighter packaging materials. Carpenters have known ________(51)centuries that some woods are tougher than others.

A in

B since

C for

D at

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2013年職稱(chēng)英語(yǔ)考試綜合類(lèi)每日一練3.16

（責(zé)任編輯：中大編輯）

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