A、By using bicycles less. B、By raising more cows. C、By cutting down trees. D、By planting more trees. D推断题。文章最后号召大家寻求办法阻止沙漠扩张，而在此

There are deserts all over the world. Over half of Africa is desert land. There are deserts in China and Latin America as well. The United States has deserts in the southwest part of the country. In some places, deserts are growing. This is a serious problem. Deserts destroy farmland. They also ruin land where animals live. When people cannot grow food or find animals to eat, [33] they have to leave their homes.
     Deserts spread for different reasons. Sometimes nature causes this problem. Wind can move sand away from deserts and onto useful land. When there is no rain for a long time many plants can die. However, humans can also cause deserts to grow. This is called desertification. This happens in many ways.  [34] One way is when people cause top much air pollution, which can make an area hotter: Hotter weather can reduce the amount of rain. Also, too many people in one area can damage the land. Especially if they have many animals, their animals can also harm the land. When large animals like cows walk on soil too much, they turn it into dust. The wind easily blows this dust away. Trees help hold water in the ground. When people cut down too many trees, the water goes away and the soil is ruined. Plants may stop growing in these places. All of these things can speed up desertification. To stop deserts from growing, people must think of ways to treat the land better.