单项选择题

What's the talk mainly about

A．The advantages of an economy based on farming.
B．Reasons farmers continued using river transportation.
C．The role of cotton in the United States economy.
D．Improved methods of transporting farm crops.

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单项选择题

What is the function of the sensory organs that the professor discusses

A．They detect nerve impulses transmitted to a grasshopper's legs.
B．They sense how far a grasshopper has jumped.
C．They detect changes in air pressure.
D．They help a grasshopper find foo

单项选择题

According to the professor, what cause the growth of the US economy about 200 years ago

A．The new technology used to build roads.
B．The ability to transport goods over land.
C．The trade in grain and cotton.
D．The linking of smaller local roads into one long roa