阅读理解
Since the earliest times, trees have always been very useful and important to human beings who made use of them in many ways. Even today trees continue to serve us m so many ways.
Trees provide man with fruits and building materials in the form of wood. Without trees itwould be
impossible to build houses, boats and even bridges. Furniture such as tables, chairs andbeds is made of
wood.
In the tropics (热带地区) where it is very hot all through the year, trees protect man fromthe terrible
heat. They are also useful in preventing good and rich top soil from being washed away during heavy rains which are so common in the tropics.
If there were no tree or vegetation of some kinds to hold back the soil with their roots, heavy rains
would wash away the rich soil that is so important and necessary to agriculture(农业). The result is that
the land becomes a desert.
There are so many desert areas in the world. A long time ago these desert areas must havebeen very
rich fertile(肥的) areas, but since our ancestors in the distant past do not have enough knowledge about
science of nature, they cut down all the trees in the areas where they lived and never planted new ones.
By and by, the rich soil was blown and washed away by strong winds and heavy rains. In the end the land was turned into deserts where nothing could grow.
1. According to the passage .
A. ancient men didn't know how to make use of trees
B. trees are not as useful now as they used to be
C. trees were more useful in the past than they are now
D. human beings have always found trees very useful
2. The underlined phrase "top soil" means .
A. useless soil which is of no use for agriculture
B. soil on the surface of the earth which is good for agriculture
C. soil found under the earth which is under roots of trees
D. dry soil in desert areas which is bad for agriculture
3. According to the passage, some desert areas were once
A. lands with good soil
B. covered by ice and snow
C. very cold
D. very dry and useless
4. If there were no trees, .
A. the land would become more fertile
B. heavy rains would clean the tropics
C. the rich top soil couldn't be held back
D. there would be no agriculture at all
5. From the passage, we know that .
A. man can do nothing to keep the balance of nature
B. man may take no notice of the balance of nature
C. man must do his best to keep the balance of nature
D. the balance of nature is unimportant to man
，紫外线在真空中的波长是λ＝c/ν≈3.7×10－7 m．在膜中的波长是λ′＝λ/n≈2.47×10－7 m，故膜的厚度至少是1.23×10－7 m.