搜题请搜索小程序“爱下问搜题”
问题 单项选择题

两直线的三组同面投影都相交，且每两面投影的交点的连线都垂直与相应的投影轴，则这两直线在空间（）。

A.一定相交

B.一定交叉

C.可能交叉

D.可能相交

答案

参考答案：A

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 "themselves" in Paragraph 1 refers to________.

A．plant physiologists

B．solutions

C．laboratory flasks

D．roots
查看答案
单项选择题

尽管诸多研究者们总结了多种教学设计过程模式，这些模式的内容虽然不尽相同，但其基本要素是相似的，都是（）。

A．分析、设计、开发、实施和评价五个要素

B．分析、设计、开发和实施四个要素

C．分析、设计、开发、实施、评价和反思六个要素

D．分析、设计和实施三个要素
查看答案