A、It can prevent global warming. B、It can catch blowing sand. C、It can stop desertification. D、It forces farmers leave cropland.

Desertification is a process. It changes productive land into useless land. One example of desertification is when a desert spreads into nearby cropland. In time, the cropland becomes an extension of the desert. But that is not the only way farmers lose fertile soil. Long dry periods, warmer temperatures and the removal of trees can all lead to the loss of good cropland. Floods can remove fertile topsoil and begin a process resulting in the loss of planting areas.
    Another danger to good land is poor farming methods. Farmers should avoid continually planting crops in the same places, or letting animals feed year after year on the same lands.
    Countries from Guatemala to Greece to Vietnam are working against the loss of cropland. Africa especially faces the risk of desertification.
    When cropland turns to desert, people move to other places for better land and better jobs. This migration can cause political and social tensions. A nonprofit organization in Nigeria is working to bring public attention to the problem. The group is called Fighting Against Desert Encroachment, or FADE. Newton Jibunoh who started this group in 2000 is currently leading a delegation to thirteen African countries to discuss the dangers of losing farmlands. In northern Nigeria, the group organized a competition between schools in seven areas. The goal was to see who could plant the most trees.
    Trees are often cut down for fuel wood. But lines of trees around cropland can catch blowing sand, In addition, tree roots can hold soil in place. Even within a desert, trees can be planted as borders around grassy areas.
19. What does the speaker say about desertification?
20. What is mentioned as one of the poor farming methods?
21. For what purpose is FADE founded?
22. What is said about the trees around cropland?