单项选择题

Scientists do not know ________.A. how children learn their own languagesB. when children begin to learn their own languagesC. why children learn their own languages
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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
单项选择题
Fires die _________.
A． without water
B． without air
C． with oxygen
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