题目内容：
根据以下材料，回答题An Essential Scientific Process
All life on the earth depends upon green plants. Using sunlight, the plants produce their own food. Then animals feed upon the plants. They take in the nutrients the plants have made and stored.
But that's not all. Sunlight also helps a plant produce oxygen. Some of the oxygen is used by the plant, but a plant usually produces more oxygen than it uses. The excess oxygen is necessary for animals and other organisms to live.
The process of changing light into food and oxygen is called photosynthesis. Besides light energy from the sun, plants also use water and carbon dioxide. The water gets to the plant through its roots. The carbon dioxide enters the leaves through tiny openings called stomata. The carbon dioxide travels to chloroplasts (叶绿题) , special cells in the bodies of green plants. This is where photosynthesis (光合作用 ) takes place. Chloroplasts contain the chlorophylls ( 叶绿素) that give plants their green color. The chlorophylls are the molecules that trap light energy. The trapped light energy changes water and carbon dioxide to produce oxygen and a simple sugar called glucose.
Carbon dioxide and oxygen move into and out of the stomata. Water vapor also moves out of the stomata. More than 90 percent of water a plant takes in through its roots escapes through the stomata. During the daytime, the stomata of most plants are open. This allows carbon dioxide to enter the leaves for photosynthesis. As night falls, carbon dioxide is not needed. The stomata of most plants close. Water loss stops.
If photosynthesis ceased, there would be little food or other organic matter on the earth. Most organisms would disappear. The earth's atmosphere would no longer contain oxygen. Photosynthesis is essential for life on our planet.
In the first paragraph, the word "excess" means___________ . A．heavy
B．extra
C．green
D．liquid
参考答案：
答案解析：