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 kill _________ every year.
A．a lot of people
B．few people
C．only some babies
单项选择题
A small fire _________.
A．is caused by a big fire
B．can become a big fire very fast
C．can never become a big fire
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