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问题 问答题

一段导体的电阻增加3Ω后，接在原电源上，发现通过该导体的电流是原来的4/5，则该导体原来的阻值是多少？

答案

设导体原来阻值为R，电源电压为U

依题意得：

U
R+3
=
4
5
×
U
R

解得R=12

答：该导体原来的阻值是12Ω．

单项选择题

As one works with color in a practical, or experimental way, one is impressed by two apparently unrelated facts. Color as seen is a mobile, changeable thing (1) to a large extent on the relationship of the color (2) other colors (3) simultaneously. It is not (4) in its relation to the direct stimulus which (5) it. On the other hand, the properties of surfaces that give (6) to color do not seem to change greatly under a wide variety of illumination color, usually (but not always) looking much the same in artificial light as in daylight. Both of these effects seem to be (7) in large part to the mechanism of color (8) .

When the eye is (9) to a colored area, there is an immediate readjustment of the (10) of the eye to color in and around the area (11) . This readjustment does not promptly affect the color seen but usually does affect the next area to which the (12) is shifted. The longer the time of viewing, the higher the (13) , and the larger the area, the greater the effect will be (14) its persistence in the (15) viewing situation. As indicated by the work of Wright and Schouten, it appears that, at (16) for a first approximation, full adaptation takes place over (17) time if the adapting source is moderately bright and the eye has been in (18) darkness just previously. Also, (19) of the persistence of the effect if the eye is shifted around from one object to another, all of which are at similar brightness or have similar colors, the adaptation will tend to become (20) over the whole eye.

5（）

A.creates

B.summons

C.triggers

D.introduces
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实验题

某家庭中，哥哥患了病毒性肝炎，并且传给了弟弟，请分析：

（1）导致弟弟患病的传染源是        ，弟弟很快就染上了此病，说明他是        

（2）该病的传播途径主要是       在哥哥发病（初期、中期）传染性最强．。

（3）从预防传染病的一般措施看，对哥哥采取隔离措施属于            ；对他们的饮食用具进行消毒属于                       ；禁止其亲人随便去医院探望属于                。
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