Tibet is among the most popular places for Chinese tourists. The number of travelers to Tibet has grown by 10% every year for a few years. Since July 1,2006, when the first train ran 1956 kilometers from Xining to Lhasa, more and more people have been going to Tibet. The train stops at several famous places along the way, such as Qinghai Lake, Kunlun Mountain, and the Potala Palace. Passengers can also enjoy many activities during the journey, like Tibetan dancing and Karaoke. On the train, passengers can have tea, eggs and noodles for breakfast, and fried chicken and green vegetables for lunch and dinner. Unlike most Chinese trains which have open - hole toilets, this one has special toilets which can collect the waste. There is also a special rubbish system   in the train that keeps the environment clean. All the windows on the train can protect people from the bright sunlight. TV and electrical sockets   for computers and mobile phones can be found on the train. Because there isn't much oxygen there, trains will have oxygen masks   for those who need _ . It makes passengers feel more comfortable when they have enough oxygen on the famous "roof of the world". There are also doctors on the train to make sure that all of the travelers are safe. We can learn from the passage that  _  .
flying to Tibet is impossible train to Lhasa is too clean to take train trip to Tibet is popular with travelers taking train to Lhasa causes a lot of trouble