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
Man can speak because ________.
A．he has a brain
B．he has a head
C．he has a mouth
单项选择题
Fires die _________.
A．without water
B．without air
C．with oxygen
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