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问题 单项选择题

达到电子平衡的条件是()

A．进入体积元的电子数目等于离开体积元的电子数目 

B．进入体积元的能量等于离开体积元的能量 

C．电子带入体积元的能量等于电子带出体积元的能量 

D．对于低能X射线，由于次级电子射程短，体积单位可以很小 

E．对于能量较高射线，次级电子的射程增长，测量体积不增大

答案

参考答案：C

单项选择题

在视网膜中，能产生动作电位的细胞是

A．色素细胞
B．视杆细胞和视锥细胞
C．双极细胞
D．水平细胞
E．神经节细胞
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问答题

Directions:

Suppose you are a college student on a school campus or an employee in a company. Recently you noticed that some people never paid attention to the signs on a garbage can that say it is desirable to deal with wastes separately. Now you are going to make a conservation campaign speech at a party. And your speech should include:

1) the present situation,

2) and your suggestions.

Write your letter in no less than 100 words. Write it neatly on ANSWER SHEET 2.

Do not sign your own name at the end of the letter, use "Li Ming" instead.

Do not write the address.
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