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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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.

单项选择题

How are tornadoes distinguished from whirlwinds

A．Whirlwinds rotate in a different direction than tornadoes do.
B．Tonadoes travel over land and whirlwinds over water.
C．Tonadoes are larger than whirlwinds.
D．Only tornadoes are funnel - shaped.