题目内容
Sugar is so much a part of our modern life that we only think about it when, for some reason, we cannot obtain it. It has been known to man for at least 3,000 years, but has come into common use only in modern times. Until quite recently it was considered as a medicine and as a luxury for the rich only.
Sugar is, then, very important to our civilization. But what exactly is it? Of course, most of us recognize sugar immediately as the sweet material which we put in coffee or cakes. This common form of sugar is from two plants: the sugar cane (a type of grass which grows to a height of 20 feet) and the sugar beet (which grows underground). But in fact there are many types of sugar, and the chemist recognizes hundreds of different varieties, each coming from a different source.
About 90% of the sugar is produced as food. Only 10% is used in industry for purposes other than food production. Yet sugar has great possibilities for use as the basis of chemicals. It can even be used for making plastics. In the future these potential uses will certainly be developed more than in the past. There are many reasons why we should increase the production of sugar. Most important is that it is one of the most highly concentrated energy foods. Thus sugar cane and beet produce an average of 7,000,000 calories per acre. In this way they have the advantages over potatoes which give only 4,000,000, while the figure for wheat and beans is 2,000,000 each. So three acres of land growing wheat, beans and potatoes give only slightly more energy than one acre of sugar.
1.The heading of Paragraph One may be ______.
A.What Sugar Is B.Sugar Through Ages
C.The Advantages of Sugar D.Present And Future Uses of Sugar
2.The third paragraph emphasizes (强调) ______.
A.about 90% of sugar is used as food B.only 10% is used in industry
C.sugar used for making plastics D.future uses of sugar
3.Sugar has been used widely ______.
A.for 3,000 years B.in modern times
C.until quite recently D.for 300 years
4.The main reason why we should increase the production of sugar is that ______.
A.sugar tastes sweet B.we can save plenty of land
C.we can take in more energy D.it provides much energy
BDBD
We all enjoy the colors of autumn leaves. Did you ever wonder how and why a fall leaf changes color? Where do the yellows and oranges come from? To answer those questions, we first have to understand what leaves are and what they do.
Leaves are nature's food factories. Plants take water from the ground through their roots. They take a gas called carbon dioxide from the air. Plants use sunlight to turn water and carbon dioxide into glucose, which is a kind of sugar. Plants use glucose as food for energy and as a building block for growing. The way plants turn water and carbon dioxide into sugar is called photosynthesis, which means " putting together with light. " A chemical called chlorophyll helps make photosynthesis happen. Chlorophyll is what gives plants their green color.
As summer ends and autumn comes, the days get shorter and shorter. This is how the trees "know" to begin getting ready for winter.
During winter, there is not enough light or water for photosynthesis. The trees will rest, and live off the food they stored during summer. They begin to shut down their food-making factories. The green chlorophyll disappears from the leaves. As the bright green fades away, we begin to see yellow and orange colors. Small a-mounts of these colors have been in the leaves all along. Covered up by the green chlorophyll, we just can't see them in summer.
The bright reds and purples we see in leaves are made mostly in the fall. In some trees, like maples, glucose is trapped in the leaves after photosynthesis stops. Sunlight and the cool nights of autumn cause the leaves turn this glucose into a red color. The brown color of trees like oaks is made from wastes left in the leaves.
It is the combination of all these things that make the beautiful colors we enjoy in the fall.
【小题1】The writer asked two questions in the beginning in order to .
A.persuade readers to believe something |
B.introduce the topic of the passage |
C.get the readers excited |
D.offer something to think over |
A.Trees don't change colours with seasons. |
B.Trees can still perform photosynthesis well in winter. |
C.Trees have colours like yellow and orange even in summer. |
D.Trees don't need food in winter. |
A.plants change water and carbon dioxide into sugar |
B.plants turn water and carbon dioxide into sugar with the help of sunlight |
C.plants use glucose as food for energy and growing |
D.chlorophyll is a great help |
A.Colorful trees in autumn | B.Mysteries of tree colors |
C.Do you enjoy tree colors? | D.Wonderful colors in autumn |