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Plants are very important living things. Life could not go on if there were no plants. This is because plants can make food from air, water and sunlight. Animals and men can't make food from air, water and sunlight. Animals get their food by eating plants and other animals. Therefore, animals and men need plants in order to live. This is the reason why we find so many plants around us.
If you look carefully at the plants around you, you will find that there are many kinds of plants. Some plants are large while others are small. Most plants are green.
Some plants have many small leaves. Others have only a few. There are many kinds of leaves. Leaves are of different shapes and colors.
Leaves are also different in other ways. Some leaves have long thin parts at the end, we call them leaf stalks, some have short leaf stalks, others even have no leaf stalks. The leaf edges are also different. Some leaves have smooth(平滑) edges. Some have a wave-shaped edges. Others have edges like the teeth of a saw(锯子).
中自变量x的取值范围是（ ）