这段话的主题句是文中划线部分: Tibet, or the Tibet Autonomous Region, is one of the province level ethnic autonomous regions of the People’s Republic of China, covering a greater part of the Qinghai-Tibet plateau in West China. The mysterious place known as the third pole of the earth with an average altitude of 4,000 meters above sea level, presents a vast expanse of beautiful, richly-endowed and somewhat mysterious land boasting of uninterrupted snow-capped mountain ranges, large and long rivers, steppe belts, magnificent temples and monasteries, and country roads lined with sutra flags , where the industrious and brave Tibetan people have lived since ancient times.