单项选择题

According to the passage, recycling is only possible when ______.

A．it is monitored by the government
B．different collection schemes operate
C．there is enough clean material
D．there is a small amount of clean material
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单项选择题
The problem of deciding at what point a baby's imitations can be considered as speech ______.
A．is not especially important because the change takes place gradually
B．is important because words have different meanings for different people
C．is one that can never be properly understood because the meaning of words changes with age
D．is one that should be completely ignored because children's use of words in often meaningless
单项选择题
It can be inferred from the passage that when a 7-month baby calls his or her mother mama ______.
A．he probably has known that mother should be called in this way
B．he probably does not know the sound stands for "mother"
C．he probably has already known the meaning of mama
D．it is the first meaningful sound pronounced by him
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