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A．Two to four months.
B．About three weeks.
C．At least half a year.
D．Two years or more.
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单项选择题A．15,000 meters.B．10,000 meters.C．40,000 meters.D．5,000 meters.

A．15,000 meters.
B．10,000 meters.
C．40,000 meters.
D．5,000 meters.

单项选择题A．To examine the chemical elements in the Ice Age.B．To look into the pattern of solar wind activity.C．To analyze the composition of different trees.D．To find out the origin of carbon-14 on Earth.

A．To examine the chemical elements in the Ice Age.
B．To look into the pattern of solar wind activity.
C．To analyze the composition of different trees.
D．To find out the origin of carbon-14 on Earth.