单项选择题

According to this passage, nobody knows for sure _________.

A． how people began to use fire
B． why people began to use fire
C． how to make a fire with matches
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单项选择题
Fires die _________.
A． without water
B． without air
C． with oxygen
单项选择题
We need fire because it helps us _________.
A． kill animals and burn forest
B． warm house, cook food and so on
C． build bridges
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