单项选择题

Why was the plant hemp essential to worldwide trade in the past

A．Ships' ropes were made from it.
B．Hemp was a very profitable export.
C．Hemp was used as fuel for ships.
D．Hemp was used as food for sailors.
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单项选择题
The San Andreas fault is ______.
A．an active fault system
B．a place where earthquakes have been predicted accurately
C．a place where earthquakes have been controlled'
D．the location of the Rocky Mountain
单项选择题
What can be said about the experiments at Rocky Mountain Arsenal
A．They have no practical value in earthquake prevention.
B．They may have practical value in earthquake prevention.
C．They are certain to have practical value in earthquake prevention.
D．The article does not say anything about their practical value in earthquake prevention.
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