单项选择题

There could be life on the Mars in the future if（）

A. it supported life in the past
B. certain gases are used to cool the planet
C. the atmosphere can be heated enough to grow simple plants
D. the planet’s volcanoes become as active as they were in the past

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单项选择题

According to the passage, （）

A. using wood from the olive groves in ancient Greece was a sensible solution to the fuel problem
B. using passive solar energy was more expensive than using wood in ancient Greece
C. the ancient Greeks planted new forests to replace the ones they cut down for firewood
D. the concept of using passive solar energy has been around since at least the fourth century B C

单项选择题

Which of the following is true according to the passage（）

A. New Yorkers are usually open with strangers.
B. People from the South are usually considered good hosts.
C. The friendliest Americans travel from place to place.
D. Different parts have different customs and habits.