单项选择题

The hard, out surface of the tooth is called enamel（）.

A．outside
B．appearance
C．outer
D．hiding

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单项选择题

Not much people realize that apples have been cultivated for over 3,000 years（）.

A．Not many
B．Not enough
C．Without many
D．No many

单项选择题

Although the social sciences different a great deal from one another, they share a common interest in human relationship（）.

A．move
B．differ
C．change
D．varies

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