相关：2007年12月大学英语四级考试最新预测真题参考答案解析
Model Test Two
Part I Writing (30 minutes)
Directions: For this part, you are allowed 30 minutes to write a letter. You should write at least 120 words, and base your composition on the outline given below in Chinese:
假设你是即将参加一英语考试的学生，需要几本相关参考书，请写封信给一书店销售部，你的信应包括:
1. 详细说明你想买的那本书的特点
2. 咨询付款方式
3. 确定送货时间及方式
Part II Reading Comprehension (Skimming and Scanning) (15 minutes)
Directions: In this part, you will have 15 minutes to go over the passage quickly and
answer the questions on Answer Sheet 1.
For questions 1-7, mark
Y (for YES) if the statement agrees with the information given in the passage;
N (for NO) if the statement contradicts the information given in the passage;
NG (for NOT GIVEN) if the information is not given in the passage.
For questions 8-10, complete the sentences with the information given in the passage.
The Hydrogen Economy
It seems like every day there is a new announcement in the news about automobiles powered by fuel cells. The promises are great, since fuel cells have the potential to very quickly double the efficiency of cars while significantly reducing air pollution.
At the same time, there have been news stories for decades about the problems associated with petroleum. Everything from oil spills to ozone alerts to global warming gets blamed on our dependence on fossil fuels.
These two forces are leading the world toward what is broadly known as the hydrogen economy. If the predictions are true, over the next several decades we will all begin to see an amazing shift away from the fossil fuel economy we have today toward a much cleaner hydrogen future.
Problems with the fossil fuel economy
While fossil fuels have played an important role in getting society to the point it is at today, there are four big problems that fossil fuels create:
Air pollution - When cars burn gasoline, the internal combustion engine also produces:
Carbon monoxide, a poisonous gas
Nitrogen oxides, the main source of urban smog
Unburned hydrocarbons, the main source of urban ozone
Environmental pollution - The process of transporting and storing oil has a big impact on the environment whenever something goes wrong. An oil spill, pipeline explosion or well fire can create a huge mess.
Global warming - The carbon dioxide coming out of every car's tailpipe is a greenhouse gas that is slowly raising the temperature of the planet. The ultimate effects are unknown, but it is a strong possibility that, eventually, there will be dramatic climate changes that affect everyone on the planet.
Dependence - The United States, and most other countries, cannot produce enough oil to meet demand, so they import it from oil-rich countries. That creates an economic dependence. When Middle East oil producers decide to raise the price of oil, the rest of the world has little choice but to pay the higher price.
Advantages of the hydrogen economy
The hydrogen economy promises to eliminate all of the problems that the fossil fuel economy creates. Therefore, the advantages of the hydrogen economy include:
The elimination of pollution caused by fossil fuels - When hydrogen is used in a fuel cell to create power, it is a completely clean technology. The only byproduct is water. There are also no environmental dangers like oil spills to worry about with hydrogen.
The elimination of greenhouse gases - If the hydrogen comes from the electrolysis of water, then hydrogen adds no greenhouse gases to the environment. There is a perfect cycle -- electrolysis produces hydrogen from water, and the hydrogen recombines with oxygen to create water and power in a fuel cell.
The elimination of economic dependence - The elimination of oil means no dependence on the Middle East and its oil reserves.
Distributed production - Hydrogen can be produced anywhere that you have electricity and water. People can even produce it in their homes with relatively simple technology.
The problems with the fossil fuel economy are so great, and the environmental advantages of the hydrogen economy so significant, that the push toward the hydrogen economy is very strong.
Technological Hurdles
The big question with the hydrogen economy is, "Where does the hydrogen come from?" After that comes the question of transporting, distributing and storing hydrogen. Hydrogen tends to be large and tricky in its natural gaseous form.
Once both of these questions are answered in an economical way, the hydrogen economy will be in place.
Where does the hydrogen come from?
There are two possible sources for the hydrogen:
Electrolysis of water - Using electricity, it is easy to split water molecules to create pure hydrogen and oxygen. One big advantage of this process is that you can do it anywhere. For example, you could have a box in your garage producing hydrogen from tap water, and you could fuel your car with that hydrogen.
Reforming fossil fuels - Oil and natural gas contain hydrocarbons — molecules consisting of hydrogen and carbon. Using a device called a fuel processor or a reformer, you can split the hydrogen off the carbon in a hydrocarbon relatively easily and then use the hydrogen. You discard the leftover carbon to the atmosphere as carbon dioxide.
The second option is, of course, slightly perverse. You are using fossil fuel as the source of hydrogen for the hydrogen economy. This approach reduces air pollution, but it doesn't solve either the greenhouse gas problem or the dependence problem. However, it may be a good temporary step to take during the transition to the hydrogen economy.
The interesting thing about the first option is that it is the core of the real hydrogen economy. To have a pure hydrogen economy, the hydrogen must be derived from renewable sources rather than fossil fuels so that we stop releasing carbon into the atmosphere. Having enough electricity to separate hydrogen from water, and generating that electricity without using fossil fuels, will be the biggest change that we see in creating the hydrogen economy.
Where will the electricity for the electrolysis of water come from? Currently, about 68 percent of the electricity produced in the United States comes from coal or natural gas. All of that generating capacity will have to be replaced by renewable sources in the hydrogen economy.
Right now there are several different ways to create electricity that do not use fossil fuels:
Nuclear power
Hydroelectric dams
Solar cells
Wind turbines
Geothermal power
Wave and tidal power
Co-generation
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