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瓦斯的定义是什么？

答案

参考答案：

矿井瓦斯是指从煤体和围岩中放出的，以及在生产过程中产生的各种气体，包括CH4、CO2、N2、C2H6、SO2、CO等，其中CH4是瓦斯的主要成分。

单项选择题

You can tell the age of a tree by counting its ring. But these records of a tree’s life really say a lot more. Scientists are using tree ring to learn what’s been happening on the sun’s surface for the last ten thousand years. Each ring represents a year of growth. As the tree grows, it adds a layer to its trunk, taking up chemical elements from the air. By looking at elements in the rings from a certain years, scientists can tell what elements were in the air that year. Dr. Stevenson is analyzing one element, carbon-14, in ring from both living and dead tree. Some ring go back almost ten thousand year to the end of the Ice Age. When Stevenson followed the carbon-14 levels changed with the intensity of solar burning. You see, the sun has cycles. Sometimes it bums fiercely, and at other times it is relatively calm. During the sun’s violent periods, it throw off charged particles in fast moving streams called "solar winds". The particularly interfere with the formation of carbon-14 on earth. When there is more solar wind activity, less carbon-14 is produced. Ten thousand years of tree ring show that the carbon-14 level rises and falls about every 420 years. The scientists concluded that solar wind activity must follow the same cycle.

The purpose of the scientists in studying tree ring is to ().

A. examine the chemical elements in Ice Age

B. look into the pattern of solar wind activity

C. analyze the composition of different trees

D. find out the origin of carbon-14 on earth
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单项选择题

VCM Virtual Channel Memory（VCM）中文名称是什么。（）

A、存储器

B、虚拟通道存储器

C、虚拟内存

D、通道内存
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