A、Hot during the day and cold at night. B、Cold during the day and hot at night. C、Hot day and night. D、Cold day and night. A戈壁沙漠

The Gobi Desert in Asia is one of the largest desert areas in the world.
It stretches across most of Mongolia and the north of China. It is shut in by mountains to the north, south and west. (16) In summer, it is very hot during the day, but at night the temperature falls steeply. In winter, it is very cold and there are snowstorms.
There is very little water in the Gobi Desert. (17) It rains very little and there are just a few streams and no big rivers at all. Most of the area is bare rock. In summer, a little coarse grass and some thorny bushes grow there. In some places, there is just enough grass to feed a few sheep, horses and camels.
About 1,300 years ago, a young Chinese traveler set out to cross the desert. (19) His name was Hsuan-Tsang and he was just 19 years old. His friends and the guide all turned back. They did not want to risk crossing the desert. So Hsuan-Tsang went on alone. He rode a horse which had crossed the desert many times before.
(18) Hsuan-Tsang tried to follow the tracks of camels and other animals.
He was in danger of losing his way. Several times he nearly died of thirst. Once he went for five days without water before he was saved by a sudden shower. Once, he was saved by his horse. It refused to go the way Hsuan-Tsang wanted and took him to a pool of water.
(19) Sixteen years later, Hsuan-Tsang went back to China across the desert.
Questions 16 to 19 are based on the passage you have just heard.
16. What is the weather like in the Gobi Desert in summer?
17. What do we learn about the Gobi Desert from the passage?
18. How did Hsuan-Tsang manage to find his way in the desert?
19. How old was Hsuan-Tsang when he came back to China across the desert?