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问题 问答题 简答题

数控冲压的特点有哪些？

答案

参考答案：

（1）提高了冲床的通用性；

（2）消耗能量少，并可提高铲平的精度，减少了冲压件后续加工的工作量；

（3）可采用批量生产的模具，安装调试模具的时间短，模具的寿命长，可提高生产效率；

（4）适合单件小批量生产，降低了冲压件的成本；

（5）设备投资大，材料利用率低。

单项选择题

About fifty years ago, plant physiologists set out to grow roots by themselves in solutions in laboratory flasks. The scientists found that the nutrition of isolated roots was quite simple. They required sugar and the usual minerals and vitamins. However, they did not require organic nitrogen compounds. These roots got along fine on mineral inorganic nitrogen. Roots are capable of making their own proteins and other organic compounds. These activities by roots require energy, of course. The process of respiration uses sugar to make the high energy compound ATP, which drives the biochemical reactions. Respiration also requires oxygen. Highly active roots require a good deal of oxygen.The study of isolated roots has provided an understanding of the relationship between shoots and roots in intact plants. The leaves of the shoots provide the roots with sugar and vitamins, and the roots provide the shoots with water and minerals. In addition, roots can provide the shoots with organic nitrogen compounds. This comes in handy for the growth of buds in the early spring when leaves are not yet functioning. Once leaves begin photosynthesizing, they produce protein, but only mature leaves can "export" protein to the rest of the plant in the form of amino acids.

The scientists found what the isolated roots need is________.

A．quite natural

B．sugar, minerals and vitamins

C．some rare vitamins

D．organic nitrogen compounds
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多项选择题

凝结水水质不合格带来哪些问题（）

A、锅炉受热面结垢

B、汽轮机叶片积盐

C、引起阀门卡涩

D、影响给水泵的正常工作
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