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问题 选择题

用两根等长的绝缘细丝线各悬一个小球并挂在同一点，如图所示．已知两球质量相同，当它们带上同种电荷时，推开一段距离r后处于平衡状态．若使它们的带电量都减半，则它们重新平衡时两球间距离将（　　）

A．小于

r
2

B．等于

r
2

C．大于

r
2

D．不能确定

答案

电量减小，根据库仑定律知，库仑力减小，两球间的距离减小．假设两球距离等于

r
2
，则库仑力与开始一样大，重力不变，则绳子的拉力方向应与原来的方向相同，所以两球距离要变大些．则两球的距离大于
r
2
．故C正确，A、B、D错误．

故选C．

写作题

书面表达。

      请首先阅读一位母亲的来信:

      I am really worried about my daughter, Judy. She always thinks she is too fat. She eats only a little rice and

some vegetables. And she eats no fish, beef or pork. "Cake" and "dessert" are taboo words (忌讳语). The

problem is that she is not fat at all. In fact, the doctor says she is thin and unhealthy. But she still believes being

thinner will make her pretty. I am afraid that she will be sick before she becomes only "prettier". Could you give

some advice to my daughter?

      请你根据这位母亲的要求，给Judy写信提一些建议。60-80字左右。提示词：均衡的饮食 balanced

diet；依我的观点 in my opinion；必要的 necessary；总而言之 In a word

Dear Judy,

___________________________________________________________________________________________

___________________________________________________________________________________________

___________________________________________________________________________________________

      Good luck!

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单项选择题

房屋租赁合同中，限定承租户使用房屋只能用于特定的使用功能，这类限定性规定包含在( )条款之中。

A．租赁用途
B．房屋的修缮责任
C．转租的约定
D．租凭期限
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