单项选择题

Johnson’s paper ______three processes for generating hydrogen in fuel cells.（）

A．compares
B．comparisons
C．is compared
D．was a comparison

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单项选择题

Portable, affordable, and stylish: personal listening devices have never been a more popular______.（）

A．area
B．gift
C．show
D．music

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The Museum of Natural History and Industry in Tanner, OH receives more than 40,000______each month.（）

A．visit
B．visited
C．visitors
D．visiting

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