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简述手弧平焊有什么特点？

答案

参考答案：

（1）平焊时，金属熔滴主要靠自重自然过渡，操作技术易掌握；

（2）若焊接参数和操作不当，第一层焊缝易形成焊瘤或未焊透；

（3）单面焊双面成形时，易产生穿透不均，背面成形不良；

（4）易出现熔渣、铁水分不清或熔渣超前形成夹渣。

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 word "intact" in Paragraph 2 is closest in meaning to________.

A．mature

B．wild

C．whole

D．tiny
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单项选择题

北京某繁华地段上一家店铺月租金为5万元，另一地段上相同店铺的月租金为3万元，前者多交的2万元属于（）

A、绝对地租

B、级差地租Ⅰ

C、级差地租Ⅱ

D、垄断地租
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