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A. It might have been hotter than it is today.
B. It might have been a cozy habitat for life.
C. It used to have more water than Earth.
D. It used to be covered with rainforests.
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A. Causes of sleeplessness.
B. Cross-cultural communication.
Cultural psychology.
D. Motivation and positive feelings.
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A. They attach great importance to sleep.
B. They often have trouble falling asleep.
C. They pay more attention to sleep efficiency.
D. They generally sleep longer than East Asians.
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A. By asking people to report their sleep habits.
By observing people's sleep patterns in labs.
C. By having people wear motion-detecting watches.
D. By videotaping people's daily sleeping processes.
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A. It has made remarkable progress in the past few decades.
B. It has not yet explored the cross-cultural aspect of sleep.
C. It has not yet produced anything conclusive.
D. It has attracted attention all over the world.
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