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重点讲解：
1. left behind 留下；遗留
You are dealing with the physical remains that people left behind.
你面对的是古人留下的实物残骸。
2. speed up 加速
I had already taken steps to speed up a solution to the problem.
我已经采取措施加快解决这个问题。
3. put...at risk 使...处于危险境地
The doctor must not allow the patient to be put at risk.
医生决不能置患者于危险境地而不顾。
4. outer space 外太空
Exploring outer space is a challenge to mankind.
探索外太空是一项向人类发出的挑战。
