美国学者科尔曼和莱茵沃特将美国消费者划分为几个阶层______
A．9 B．8C．7 D．6
参考答案：C
— Must we keep the windows ______ all the time?
— No, you ______.[ ]
A. closed; mustn't
B. close; mustn't
C. closed; needn't
D. close; needn't
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