A、To save water. B、To protect the soil. C、To establish the roots. D、To let the water run off the surface. A短文最后提到，有效浇水的其中一个技巧是中间

(17)Some trees can survive a long time without water. Think of trees that grow in the desert. But other trees may need more moisture than they can get from rainfall or from the air. The soil stores moisture during cooler weather. But most of that moisture is gone by the middle or end of the hottest months of the year.(18)Trees and other plants can look thirsty. Leaves hang downward and turn brown or yellow. Here are some suggestions for watering newly planted trees and established trees. With a newly planted tree, the roots have not yet spread out from the root ball. The root ball can become dry faster than the dirt around it. So put water on the area of the football and the surrounding soil until the roots become established.(19)Once a tree is well established, water deeply instead of often. During the hottest season, a deep watering may satisfy a tree for anywhere from ten days to two weeks.(20)One of the effective watering skills is to stop when the soil needs a little time to absorb the water. Otherwise the water runs off the surface and gets wasted.
17. What can we learn about trees in the desert?
18. What’s the symptom when trees look thirsty?
19. How should we water a tree after it has grown up?
20. Why should we stop watering when the soil absorbs the water?