請依下文回答第 46 題至第 50 題：China completed the construction of the Qinghai-Tibet Railroad in 2006. This 1,142-km railroad line runs acrossthe snow-covered Plateau of Tibet, effectively linking the Tibetan capital, Lhasa, to the northwestern province ofQinghai. Also known as the Pan-Himalayan Railroad, this record-breaking railroad rises to 5,072 meters, earning thetitle as the world's highest passenger train with more than eighty percent of the track 13,000 feet or 3,962 meters abovesea level.The Qinghai-Tibet Railroad is a great engineering achievement. Since it was built at high altitudes, both engineersand workers had to breathe bottled oxygen for fear of altitude sickness while working on the construction of the track.Additionally, there were many other challenges they had to cope with, including the soil that can rise and sink as theseason changes. Some sections of track had to be laid on ground that is permanently frozen. Due to global warming, therising temperatures may cause this permafrost to melt, making the land sink to a lower level. The track may becomebent or curved as a consequence. Nevertheless, some smart countermeasures were taken so that the engineers managedto cool off the roadbed by keeping the permafrost stable.The cost savings to travelers resulting from the completion and operation of the Qinghai-Tibet Railroad aretremendous. Compared with typical fares for direct air travel, a typical train fare between Beijing and Lhasa is muchcheaper. Moreover, travelers get to enjoy the magnificent sights and scenery along the way, making the long trip morethan worthwhile. It is hoped that with better linkage to the world, the tourism industry will play an increasinglyimportant role in Tibet's economy.
46 What is the main purpose of this passage?
(A) To compare two famous Chinese railroads.
(B) To explain a health hazard at high altitudes.
(C) To detail the process of constructing a railroad.
(D) To persuade travelers to take a train trip to Tibet.