单项选择题

We had experienced a period of ______ economic boom and depression.

A．discrete
B．compound
C．alternate
D．optional
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单项选择题
The public and authorities are particularly concerned about the ______ from the unclear power stations.
A．extracts
B．injections
C．discharges
D．drainage
单项选择题
When Americans tell a joke, they often ______
A．poke people in the ribs
B．pat people on the arm
C．stroke people's hair
D．give people a big hug
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