The Roman Empire was the first true superpower in human history。（)
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Charlemagne was the greatest Frankish king, who founded the first empire in Western Europe after the fall of Rome。（)
在机械通风和自然通风的烟囱各起什么作用？其高度根据什么原则确定？
The Hammurabi Code is the oldest known legal document in human history。（)
什么叫自生风？自生风的正负号如何决定？在水平烟道有没有自生风？
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