题目内容：
阅读短文，回答题。The name Sahara derives from the Arabic word for "desert" or "steppe". At 3. 5 millionsquare miles, an area roughly the size of the United States, the Sahara Desert in northern Africais the largest desert in the world. It spans the continent from the Atlantic Ocean to the Red Sea.Daytime temperatures can reach as high as 130 F. The humidity sometimes gets into the teens.But can also be as low as 2.5 percent, the lowest in the world. Most of the Sahara receives lessthan five inches of rain per year, while large areas sometimes have no rainfall at all for years.The heart of the Sahara is the landlocked north African country of Niger. Here the sand dunescan be 100 feet tall and several miles long. Here sand plains stretch over an area larger thanGermany where there is neither water nor towns. Yet sitting in the midst of the surroundingdesert is the town of Bilma. Suddenly there are pools of clear water. Surprisingly, there are
groves of date palms. Underground water resources, or oases, sufficient to support irrigatedagriculture are founding dry stream beds and depressions. Irrigation ditches run off a creek towater fields. Corn, cassava, tea, peanuts, hot peppers, and orange, lime, and grapefruit treesgrow in these fields. Donkeys and goats graze on green grass.
The Sahara of Niger is still a region where you can see a camel caravan of 500 camels tiedtogether in loose lines as long as a mile, traveling toward such oasis towns. There a caravan willcollect life-sustaining salt, which is mined from watery basins, and transport it up to 400 milesback to settlements on the edges of the desert. The round trip across the vast sands takes onemonth.
In this passage caravan means A．traveling circus
B．group traveling together through difficult country
C．railroad train
D．a small, fast sailing ship
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