问题
选择题
有X、Y两种元素，相对原子质量分别为56、16，化合价分别为+3和-2，则X、Y组成的化合物的相对分子质量为（ ）
A．72
B．88
C．160
D．200
答案
X、Y两种元素化合价分别为+3和-2，则X、Y组成的化合物的化学式为：X2Y3；X、Y两种元素的相对原子质量分别为56、16，其相对分子质量为56×2+16×3=160．
故选C．
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有X、Y两种元素，相对原子质量分别为56、16，化合价分别为+3和-2，则X、Y组成的化合物的相对分子质量为（ ）
A．72
B．88
C．160
D．200
X、Y两种元素化合价分别为+3和-2，则X、Y组成的化合物的化学式为：X2Y3；X、Y两种元素的相对原子质量分别为56、16，其相对分子质量为56×2+16×3=160．
故选C．
Radioactive(放射性的) pollution can be said to be the emission(排放) of radioactive substance into air, water or land due to human activities in the form of radioactive waste. Radioactive waste is usually the product of nuclear fission(核裂变), which is widely used in nuclear power stations and nuclear weapons. The radioactivity of nuclear waste decreases with time. That means the waste needs to be separated from the reach of living beings until it no longer causes a threat to living beings. This time period may take from days to months and to years depending upon the radioactive nature of the waste. Radioactive pollution that is spread through the earth’s atmosphere is called ‘fallout’. Nuclear pollution began to attract people’s attention during the World War II period when the United States, Britain and the Soviet Union started conducing nuclear tests. The best example of fallout is the nuclear bomb attack on Hiroshima and Nagasaki, Japan in 1945 by the United States of America during World War II. As a result of the bomb attack, nearly 225,000 people died within 5 years of the attack due to the radiation effects and cancer. In land and water, the major source of radioactive pollution remains with the nuclear fuel cycle(核燃料循环). The nuclear fuel cycle is used in nuclear power stations and extraction(提炼) of materials from nuclear substance, where the pollutants are left behind after the useful material has been extracted. The effects of radioactive pollution were first reported in the early 20th century when people working in uranium mines(铀矿井) suffered from skin burn and cancer. The effects vary from one individual to another. Exposed to radioactive radiations for a long time, living beings may suffer from the damage to the DNA cells, which results in cancer, genetic defects for the generations to come and even death. In a word, since every coin has its two sides, it’s a good idea to prevent from its disadvantages while making full use of its advantages.
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