题型：任务型阅读 题类：常考题 难易度：普通
陕西省商洛市2019~2020学年高二下学期英语期末考试试卷
Trees are commonly cut down for wood and paper. . The unique chemical process that trees and plants use to turn light energy from the sun into oxygen is known as photosynthesis (光合作用).
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The production of oxygen is a result of photosynthesis, but it is not the main purpose of this process. . Plants create their own food through photosynthesis. During this process, a plant's roots absorb water from the ground, and its leaves take in light energy and carbon dioxide. The plant uses these elements to make fats and proteins that are then used to maintain the plant's life.
The process of photosynthesis
The first step in photosynthesis is using the sun's energy. During this process, chlorophyll (叶绿素) absorbs the light energy of the sun. The energy from the sun then acts upon the water absorbed by the roots of the plant. Carbon dioxide sent into the atmosphere by animals and humans is then absorbed by the plant's leaves and paired with the hydrogen to produce sugar. The sugar is turned into plant food. .
Threats to tree photosynthesis
. Today, only about 30 percent of Earth's land is covered in trees. This is contributing to global warming since trees are necessary to consume the carbon dioxide in the atmosphere. Experts believe planting trees is vital in order to ensure the preservation of the delicate balance that enables photosynthesis.
A. The purposes of photosynthesis
B. The advantages of photosynthesis
C. In fact, oxygen is simply a by product
D. It is also responsible for giving plants their green color
E. During this
process 
extra oxygen is produced and released
F. The trees that change carbon dioxide into oxygen are disappearing rapidly
G. But the greatest value of trees comes from their ability to turn the sun's energy into chemical energy
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