单项选择题

Ammonia, one of the earliest known nitrogen compounds, was originally produced by distilling organic materials.

A. masses
B. fabrics
C. substances
D. liquids
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单项选择题
The climate and soil as far north as the Arctic Circle permit farmers to raise livestock and grow barley, potatoes and other crops.
A. help
B. allow
C. require
D. expect
单项选择题
In the United States, the national flag is usually displayed outdoor between sunrise and sunset.
A. stored
B. folded
C. respected
D. exhibited
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