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
The professor is quite difficult pleased .（）

A．to please
B．to be pleased
C．for pleasing
D．pleasing
单项选择题
Unlike competitive running , race walkers must always keep some portion of their feet in contact with the ground.（）

A．run
B．runner
C．runners
D．running race
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