B. Read the passage carefully, then choose the correct answer.

Most people think of deserts as dry, flat areas with little vegetation and little or no rainfall, but this is hardly true. Many deserts have varied geographical formations ranging from soft, rolling hills to stark, jagged cliffs, and most deserts have a permanent source of water. Although deserts do not receive a high amount of rainfall - to be classified as a desert, an area must get less than twenty-five centimeters of rainfall per year - there are many plants that thrive on only small amounts of water, and deserts are often full of such plant life.

Desert plants have a variety of mechanism for obtaining the water needed for survival. Some plants, such as cactus, are able to store large amounts of water in their leaves or stems; after a rainfall these plants absorb a large supply of water to last until the next rainfall. Other plants, such as the mesquite, have extraordinarily deep root systems that allow them to obtain water from far below the desert's arid surface.

What is the main topic of the passage?