电源屏的输出闪光电源，通断比为1：1，其闪光频率为多少？

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问题问答题简答题

答案

参考答案：

闪光频率在室内作表示使用时，为90－120次/min；在室外作信号点灯用时，为50-70次/分钟。

不定项选择

在真空中A、B两点分别放置等量异种电荷，在电场中通过A、B两点的竖直平面内对称位置取一个矩形路径abcd，如图所示，现将一电子沿abcd移动一周，则下列判断正确的是

A．由a→b电场力做正功，电子的电势能减小

B．由b→c电场对电子先做负功，后做正功，总功为零

C．由c→d电子的电势能一直增加

D．由d→a电子的电势能先减小后增加，电势能总增加量为零

填空题

仔细阅读下面短文并按照语境完成填空 (每空只能填一个单词)，注意语法，词性、词形变化等。

Dear Dr Health,

I think too many people take weight-loss pills without really knowing that they can 1._______ their

health.

I read about a Canadian actress 2._______ had to go to hospital because she took some weight-loss

pills. She lost 7 kg in two months. However, the pills 3._______ something that causes liver failure.

Recently, my cousin read about a weight-loss pill and she really wants to try it. I've told her the story

of the 4. _______, but she won't listen. She's only 12, but has become a girl who is worried 5. _______

her figure and how she looks. She often refuses to eat. My aunt is worried that my cousin will buy the

pills in secret. She says health is priceless. She thinks that children must eat properly 6._______ they are

growing all the time.

What am I to do? Please help me.

Yours truly,

Xiao Ming