Step by Step 3000 第1册 Unit12:Dictation item(8)
时间：2015-05-18 05:56:50
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（单词翻译）
Item eight.
Petroleum3 has been important since ancient times.
In Latin, the name means "rock oil".
Petroleum is a fossil fuel.
The liquid comes from the
remains4 of plants and animals that died millions of years ago.
These remains were buried deep
bellow5 levels of rock over time and under great pressure.
This geological process created complex
molecules6 of hydrogen and carbon.
Oil can also contain other elements.
Crude oil or unprocessed petroleum is called sour when it contains a lot of sulfur and
impurity7.
Sour crude requires more refining than sweet crude which is often more valuable.
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