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问题 选择题

铜的密度是8.9×103kg/m3下列说法不正确的是（　　）

A．lm3铜的质量为8.9×103kg

B．一个铜块切割成体积相等的两块后，密度仍是8.9×103kg/m3

C．将一个铜块加热，密度仍是8.9×103kg/m3

D．铜的密度比铝的密度大，所以体积相同的铜块比铝块质量大

答案

A、由密度定义可知：铜的密度是8.9×103kg/m3，表示lm3铜的质量为8.9×103kg，故本选项正确，但不符合题意；

B、由ρ=

m
V
得，因为锯去一半，剩下一半的质量和体积都减去一半，根据ρ=
m
V
可得，其密度仍然是8.9×103kg/m3．故本选项正确，但不符合题意；

C、将一个铜块加热，其体积变大了，所以根据密度公式ρ=

m
V
可知，其密度变小了，故本选项错误，但符合题意；

D、质量相同，由m=ρV知，体积越大密度越小．体积相同，由 V=

m
ρ
知，质量越小密度越小，故本选项正确，但不符合题意．

故选C．

单项选择题

防水服（保温）的穿着时间为（）

A、≤1分钟

B、≤2分钟

C、≤3分钟
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填空题

A Thirsty Planet


If you live in a city in North America or Europe, you have probably never thought much about water. Whenever you need some, you turn on the tap and there it is. Millions of people in other parts of the world are not so lucky. They have trouble getting enough clean water for their basic needs. This situation may soon become common all around the world, scientists believe. In fact, they say that the tack of clean water may be one of the biggest issues in the twenty-first century.
The reasons for this are clear. On the one hand, people are using more water than ever before. Over the last fifty years, the population of the world has more than doubled. So has the demand for water — for home use, for farming and for industry. On the other hand, supplies of clean water are disappearing. Many sources of surface water — such as rivers, lakes and streams — are too polluted and unhealthy for use as drinking water. This has forced more and more people to drill wells so they can get water from underground.
There are enormous amounts of water deep underground in lakes called aquifers. Until recently, scientists believed this groundwater was safe from pollution. Then, in 1980s, people in the Untied States began to find chemicals in their well water, and scientists took a closer look at what was happening. Weldon Spring, Missouri, for example, was the site of a bomb factory during World War Ⅱ. The factory was destroyed after the war, but poisonous chemicals remained on the ground. Very slowly, theses chemicals dripped down through the ground and into the aquifer. Once they did, however, the water from that aquifer was no longer drinkable.
It probably never will be drinkable again. Groundwater is not renewed regularly by the rain, like lake or river water. Thus, if a harmful chemical gets into an aquifer, it will stay there for a very long time. Furthermore, it is nearly impossible to remove all the water in an aquifer and clean out the pollutants.
Industrial sites like Weldon Spring are one cause of groundwater pollution. There are thousands of such sites in the Untied States alone, and many others around the world. Groundwater pollution is also caused by modem farming methods, which require the use of large amounts of chemicals in the fields. And finally, yet another important cause of groundwater pollution is waste. That includes solid waste thrown away in dumps and landfills, and also untreated human and animal waste.
The situation is indeed very serious. Fortunately, there are many aquifers and they are very large. Only a small number have been seriously damaged so far. But if the world does not want to go thirsty in the near future, further pollution must be prevented. Around the world, governments must make real changes in industry, agriculture and waste disposal.

We can infer from this passage that industry, farming and waste are ______.
A. the three main sources of groundwater pollution
B. minor sources of ground water pollution
C. causing problems in bomb factories
D. polluting all aquifers in the United States
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