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问题 选择题
设集合A={1，2，3，4}，m、n∈A，则方程
x2
m
+
y2
n
=1表示焦点位于x轴上的椭圆有（　　）
A．6个B．8个C．12个D．16个
答案

焦点位于x轴上的椭圆则，m＞n

当m=2时，n=1；当m=3时，n=1，2；当m=4时，n=1，2，3；

共6个

故选A．

问答题 简答题

什么叫塔板的操作弹性？
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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 underlined word "themselves" in Paragraph 1 refers to ______.

A．plant physiologists

B．solutions

C．laboratory flasks

D．roots
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