问答题

‎在机床上,发生线是由刀具的切削刃与工件间的相对运动得到的。由于使用的刀具切削刃形状和采取的加工方法不同,形成发生线的方法有4种:(1)       ;(2)       ;(3)       ;(4)       。​

答案: 轨迹法、成形法、相切法、展成法
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Should We Discourage People from Using Cars?
Nowadays the fast increase in the number of cars has brought about many problems, which are worth our greatest attention. What are those problems? In order to solve the problems, should we discourage people from using cars?
    First, urban traffic is getting from bad to worse. I’d like to use my own experience as an example. Five years ago, it took me about 30 minutes to drive from home to office. One year later, I needed to spend about 40 minutes on the way. Then two years later, I had to start out from home at least an hour earlier in order to get to my office on time for work. And then, since last year, it has just been too normal for me to spend about one and a half hours to make it. The traffic moves just like a worm! 
Second, with the ever-growing number of cars, air quality in urban areas is becoming worse over time. We used to see clear blue sky and breathe fresh air, but now this might be a real luxury. Car exhaust has seriously polluted the air and people’s health is greatly affected. It seems certain that we human beings have already made a fatal mistake that could only be corrected by greatly reducing the number of cars.
Though we can not deny the fact that cars have brought us speed, comfort and greater mobility, these have already been history. We are suffering much more from cars than benefiting from them. Our health conditions are going down rapidly due to both the polluted air that we have to breathe every second and our lack of exercise that results from the increasing amount of time we have to spend driving.
So what can we do? In my opinion, we have already found a solution: discourage people from using cars! We know very well that science and technology is also developing very fast and there might be better solutions coming up in the future, but, before that, let’s first save ourselves.
 
When did the author spend about 30 minutes to drive from home to office? A.
Two years ago. B.
Three years ago. C.
Four years ago. D.
Five years ago.

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