汽车、拖拉机的齿轮要求表面高耐磨性，中心有良好的强韧性，应选用（）。
A. 20钢渗碳淬火后低温回火
B. 40Cr淬火后高温回火
C. 20CrMnTi渗碳淬火后低温回火
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65、65Mn、50CrV等属于（）类钢，其热处理特点是（ ）
A. 工具钢，淬火+低温回火
B. 轴承钢，渗碳+淬火+低温回火
C. 弹簧钢，淬火+中温回火
下列材料是合金工具钢的是（ ）
A. 09Mn2V
B. T12
C. 45
D. 9SiCr
Which of the following statements is/ are not true according to the video?
A good story usually has a core moral.
B. A good story attract every one.
C. When telling a story, we must make it simple.
D. When telling a story, we can start with personal experiences.
E. We should avoid difficulties in the story.
F. Make yourself the hero in your story because the audience like it.
[音频]a group of related animals or plans that is smaller than a genius
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