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
。