布朗太太走了进来，告诉全班同学下星期他们必须更换教室。
Mrs. Brown came in and told the whole class they would have to change the classroom the next week.
他们说，有一天，对于现在必须开胸更换病变的脉瓣的低风险患者，这一新的过程可能是一种替换选择。
Someday, they say, the new procedure may be an alternative for the kind of lower-risk patients who now have to have their chests opened to replace diseased aortic valves.
在封井过程中，他们必须把用于在钻探过程里，保持钻井与海水压力相当的高密钻井泥更换掉：因为上方的海水的压力低于底下油气的压力。
As part of that process they had to replace the dense drilling mud, which was used to keep the well under pressure while it was drilled, with sea-water.
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