All the useful energy at the surface of the earth comes from the activity of the sun. The sun heats and feeds creatures and mankind. Each year it provides men with two hundred million tons of grain and nearly ten million tons of wood, coal, oil, natural gas, and all other fuels are stored energy from the sun. (80) Some was collected by this season’ s plants as carbon compounds. Some was stored by plants and trees ages ago. Even waterpower derives from the sun. Water turned into vapor by the sun fails as rain. It courses down the mountains and is converted to electric power. Light transmits only the energy that comes from the sun’ s outer layer, and much of this energy that is directed towards the earth never arrives. About nine tenths of it is absorbed by the atmosphere of the earth. In fact, the earth itself gets only one half millionth of the sun’s entire output of radiant energy.
The sun is the source of all of the following EXCEPT（）
The surface of the Earth is the shore of the cosmic ocean. From it we have learned most of what we know about space. Recently, we have waded a little out to sea, enough to dampen our toes or, at most, wet our ankles. The water seems inviting. The ocean calls.
The dimensions of the cosmos are so large that using familiar units of distance, such as meters or miles, chosen for their utility on Earth, would make little sense. Instead we measure distance with the speed of light. In one second a beam of light travels 186,000 miles, nearly 300,000 kilometers, or seven times around the Earth. In eight minutes it will travel from the Sun to the Earth. We can say the Sun is eight light-minutes away. In a year, it crosses nearly ten trillion kilometers, about six trillion miles, of intervening space. That unit of length, the distance light goes in a year, is called a light year. It measures not time but distances, enormous distances.
The Earth is a place. It is by no means t
A. The Cosmic Ocean
B. Sunbeams
C. The Speed of Light
D. Random Numbers
The surface of the earth may seem very stable to you. But you might be amazed if you knew some of the things that are going on under that surface.
The earth has an outer shell of rigid pieces called tectonic plates (地壳构造板块). The plates include both ocean floor and dry land. Some have whole continents on top of them. The continents on top of the plates are just going along for a slow ride, moving only about four inches per year. But even this small movement causes three types of big interactions.
One type is ocean ridges. These ridges develop in places where two plates are moving away from each other. As the plates separate, hot magma (岩浆) flows up to fill the space. New crust (地壳) builds up on the plate boundaries and causes ocean ridges. These ridges form long mountain ranges, which only rise above the ocean surface in a few places.
Another type of reaction-trenches-occurs between two
A. "a place for horses"
B. "calm and easygoing"
C. "steady or firm"
D. "a collection of animals"
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