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问题 填空题

有一电铃，它的阻值是10Ω，在正常工作时它两端的电压是6V，要把它接在电压是9V的电源上时，则必须给它______联一个______Ω的电阻．

答案

要使电铃正常工作，需要串联一个电阻来分压；

电铃正常工作时的电流I=

U电铃
R电铃
=
6V
10Ω
=0.6A，

电阻两端的电压UR=U-U电铃=9V-6V=3V，

∵I=

U
R

∴串联电阻的阻值为：

R=

UR
I
=
3V
0.6A
=5Ω．

故答案为：串；5．

填空题

以下程序的输出结果是 【8】 。 #include＜iostream＞ using namespace std； int main(){ int sum，i； for(sum=0，i=1；i＜5；i++)sum+=i； cout＜＜sum＜＜endl； return 0； }
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单项选择题

Most plants can make their own food from sunlight, (1) some have discovered that stealing is an easier way to live. Thousands of plant species get by (2) photosynthesizing, and over 400 of these species seem to live by pilfering sugars from an underground (3) of fungi(真菌). But in (4) a handful of these plants has this modus operandi been traced to a relatively obscure fungus. To find out how (5) are (6) , mycologist Martin Bidartondo of the University of California at Berkeley and his team looked in their roots. What they found were (7) of a common type of fungus, so (8) that it is found in nearly 70 percent of all plants. The presence of this common fungus in these plants not only (9) at how they survive, says Bidartondo, but also suggests that many ordinary plants might prosper from a little looting, too.

Plants have (10) relations to get what they need to survive. Normal, (11) plants can make their own carbohydrates through photosynthesis, but they still need minerals. Most plants have (12) a symbiotic relationship with a (13) network of what are called my corrhizal fungi, which lies beneath the forest (14) . The fungi help green plants absorb minerals through their roots, and (15) , the plants normally (16) the fungi with sugars, or carbon with a number of plants sharing the same fungal web, it was perhaps (17) that a few cheaters—dubbed epiparasites—would evolve to beat the system. (18) , these plants reversed the flow of carbon, (19) it into their roots from the fungi (20) releasing it as "payment.\

5（）

A. others

B. the others

C. other

D. the other
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