专四专八考试

解析：What is the passage mainly about

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【单选题】What is the passage mainly about


A．The exhaust system of a car.
B．Waste gas.
C．The car engine.
D．Air pollution.


网考网参考答案：A
网考网解析：
[听力原文]14-17 Many people tend to disregard the exhaust system of an automobile. They may think that the exhaust system does nothing more than to discharge the waste products of the combustion process. Although this is exactly what the exhaust system does, the way these waste product is discharged out of the engine is essential to the performance of the engine. A gasoline engine involves three processes: combustion, mechanical drive and exhaust. If one of these processes is impeded, the condition of the engine will depreciate. The exhaust system has three major functions, the first of which is to channel out the waste products of combustion out of the engine to allow it to continue to burn fuel unhindered thereby ensuring that the engine runs smoothly. An efficient exhaust system allows the waste gas and compound to escape quickly out of the system. If these waste products do not escape from the engine immediately, they may choke up the engine thus causing it to break down. Another function of the exhaust system is to reduce the noise generated by the engine. The third function of the exhaust system is to clean up the emissions that are harmful to the environment. When the engine burns fuel, it produces gases that pollute the air such as hydrocarbons, carbon monoxide and nitrogen oxides. document.getElementById("warp").style.display="none"; document.getElementById("content").style.display="block"; 查看试题解析出处>>

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