单项选择题

Scientists do not know ________.

A．how children learn their own languages
B．when children begin to learn their own languages
C．why children learn their own languages
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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
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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