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问题 单项选择题

切丝机刀门气缸（油缸）旁边的位移传感器是一种（）元件，当位移传感器感受到长度的变化时，会向控制系统输出信号，控制面板上显示出新的刀门高度值。

A.控制

B.检测

C.驱动

D.显示

答案

参考答案：B

问答题

随着我国家电下乡政策的落实，越来越多的家用电器进入到了我们的家庭，改善了我们的生活。请你利用学过的物理知识回答下列问题：

（1）在家庭电路中，洗衣机、电冰箱、电灯等用电器之间的连接方式是怎样的？

（2）用来连接洗衣机、电冰箱的电源插座为什么要用带有接地线的三孔插座？

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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 free 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 withorganic 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 use of the phrase "comes in handy" underlined in Paragraph 2 indicates that the process is ______.

A．unavoidable

B．predictable

C．necessary

D．successful
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