单项选择题

According to the passage, some tornadoes take people unaware because they ______.

A．produce a sound similar to thunder
B．cannot be seen until it is too late
C．look so much like other clouds
D．carry so many large pieces of debris
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单项选择题
The author refers to straw embeded in fence posts in order to show ______.
A．where tornadoes occur
B．who makes reports about tornadoes
C．when tornadoes take form
D．how tornadoes affect physical objects
单项选择题
What is the main purpose of the passage
A．To outline contrasting types of economic systems.
B．To explain the science of economics.
C．To argue for the superiority of one economic system.
D．To compare barter and money-exchange markets.
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