Choose the best word to complete the passage below.

There are deserts all over the world. They can be found (23) ________ Africa, China, South America, and North America. In some places, deserts are growing. This is a serious problem because deserts destroy farmland and ruin the land where animals live. When people cannot grow food or find animals to eat, they have to leave their homes.

Sometimes, (24) ________ can cause deserts to spread. Wind can move sand away from deserts and onto useful land. When there is no rain for a long time, plants die and deserts grow. However, humans can also cause deserts to grow. This is called desertification, and it happens in many ways. One way is when people cause too much air pollution, which can make an area hotter. Hotter weather can reduce the amount of rain as well. Too many people in one area can also damage (25) ________ land. In addition, having many animals can 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 (26) ________ many trees, less water stays in the ground, and the soil is ruined.

All of these things can speed up desertification. To stop deserts from growing, people must think of ways to treat the land (27) ________.

Choose the letter A, B, C, or D to indicate the correct sentence.