问题 选择题

下列说法正确的是(  )

A.任何化学反应均遵循质量守恒定律

B.镁带燃烧,质量增加,不符合质量守恒定律

C.蜡烛燃烧,质量减少,不符合质量守恒定律

D.根据质量守恒定律,2g氢气与8g氧气反应,应生成10g水

答案

A、化学变化是反应物分子分解成的原子进行重新结合的过程,该过程中原子种类、个数都不变,因此,任何化学反应均遵循质量守恒定律;故A正确;

B、镁带燃烧是镁与氧气反应生成氧化镁,根据质量守恒定律,生成物氧化镁的质量等于参加反应镁与氧气的质量和,因此质量增加,这正是质量守恒的结果;故B不正确;

C、蜡烛燃烧是蜡烛与氧气反应生成了二氧化碳和水扩散到空气中,由于生成物不易察觉而感觉质量减少,这也是质量守恒的结果;故C不正确;

D、根据氢气与氧气反应的化学方程式,可知反应中氢气与氧气质量比为1:8,即8g氧气只能支持1g氢气燃烧,反应后生成9g水,而不是10g水,因此该说法违背了质量守恒定律;故D不正确;

故选A.

单项选择题

Large parts of the world have not enjoyed the remarkable global progress in health conditions that have taken place over the past century. Indeed, millions of deaths in impoverished nations are avoidable with prevention and treatment options that the rich world already uses. This year, 10 million children will die in low and middle income countries. If child death rates were the same as those in developed countries this figure would be lower than 1 million. Conversely, if child death rates were those of rich countries just 100 years ago, the figure would be 30 million. Today’s tools for improving health are so powerful and inexpensive that health conditions could be reasonably good even in poor countries if policy makers spent even relatively little in the right places.

Recent research for the Copenhagen Consensus identifies several highly cost-effective options that would tackle some of the planet’s most urgent health problems. The most promising investment is in tuberculosis treatment. Some 90 percent of the 1.6 million tuberculosis deaths in 2003 occurred in low-and middle-income countries. Because tuberculosis affects working-age people, it can be a trigger of household poverty. The cornerstone of control is prompt treatment using first-line drugs, which doesn’t require a sophisticated health system. Spending $1 billion on tuberculosis treatment in a year would save 1 million lives. Because good health accompanies higher levels of national economic welfare in the long run, the economic benefits are worth $ 30 billion.

The second most cost-effective investment is tackling heart disease. Heart disease might not seem like a pressing issue for poor nations, but it represents more than a quarter of their death toll. Measures to reduce risk factors other than smoking — high intake or saturated animal fat, obesity, binge drinking of. alcohol, physical inactivity, and low fruit and vegetable consumption — have had little success. Treating acute heart attacks with inexpensive drugs is, however, cost-effective. Spending $ 200 million could avert several hundred thousand deaths, yielding benefits that are 25 times higher than costs.

The third option is prevention and treatment of malaria. A billion dollars would expand the provision of insecticide-treated bed-nets and facilitate provision of highly effective treatment. This would save more than a million child deaths and produce economic benefits worth $ 20 billion.

The fourth alternative for policymakers is to focus on child health initiatives. The best measures are familiar ones expanding immunization coverage, promoting breastfeeding, increasing the use of simple and cheap treatments for diarrhea and childhood pneumonia, and so on.

Even if the costs of all these initiatives were two or three times higher than we estimate, these efforts would still provide amazing opportunities to reduce health inequality and do good in the world.

The most effective investment is in the treatment for().

A.tuberculosis

B.heart disease

C.malaria

D.diarrhea

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