Понимание структурно-смысловых связей в тексте

The roof of the world

Did you know that Tibet is called the “roof of the world?” Tibet is a small country surrounded on all sides by gigantic snowy mountain peaks. For thousands of years, these towering mountains acted like a fence, A __________. That’s one reason why explorers and writers have called Tibet the roof of the world. It’s hard to get to. The other reason is Tibet’s high elevation. When people climb mountain passes over 17,000 feet above sea level, they gasp for air B __________!
Years ago, the people of Tibet were nomads, C __________. The ground in Tibet is much too rocky and thin to grow crops, so Tibetans centred their daily life and survival on the large ox, the yak. The yaks provided the nomads with nearly everything they needed – D __________. Even yak dung was used for fires.
Tibetan nomads would lead their herds of yaks and sheep across pastures, valleys, and mountainsides in search of the best grazing lands. They did not live in permanent homes made of wood, brick, or stone. When nomads arrived at their destination, they were so skilled at setting up their large yak-hair tents that they had them up in minutes. They could even compete to see E __________, a fire going, and hot tea poured.
Times are changing in Tibet, and more and more people live and work in villages and cities. But there are still nomads F __________.

1. which means people without permanent homes
2. who would be the first one to have their tent up
3. as they may need 30 yaks to carry their supplies
4. milk, butter, meat, and wool for clothes and ropes
5. as they are more than three miles high above the sea
6. keeping people from entering or leaving the country
7. that survive in the mountains just as their ancestors did