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问题 单项选择题

如果随机变量X与Y满足D（X+Y）=D（X-Y），则下列结论中正确的是（）。

A. X与Y独立

B. D(Y)=0

C.D(X)·D(Y)=0

D. X与Y不相关

答案

参考答案：D

解析：

因为D(X+Y)=D(X-Y)，即协方差σXY=0，所以X与Y不相关。

阅读理解

阅读理解。   

        Mrs Green is going to give her daughter, Mary, a birthday party. Mary will be ten years old. A lot of her friends are coming to the party. Twenty of them are girls.

       Mrs Green is getting ready for the party. Mrs White is helping her.

      "That's a big nice cake," says Mrs White.

      "Thank you very much. "

      Mrs Green has bought a lot of pears, apples, oranges and bananas.

      It's five o'clock in the afternoon. Everything is ready. Now the first girl is arriving. The party is going to begin in thirty minutes.

根据短文内容,回答21-25问题。

1. Who is Mrs Green going to give a birthday party to?

                                                                                           

2. Who is helping Mrs Green get ready for the party?

                                                                                           

3. What fruit did Mrs Green buy?

                                                                                           

4. When is the party going to begin? 

                                                                                           

5. How many girls will be there at the party? 

                                                                                           

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单项选择题

地上立式油罐的管壁到防火堤堤脚线的距离，不应小于（）。

A、油罐的直径

B、油罐的半径

C、油罐壁高度的一半

D、油罐壁的高度
查看答案