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问题 单项选择题

等式Var(X+Y)=Var(X)+Var(Y)成立的条件是( )。

A．X与Y同分布

B．X与Y同均值

C．X与Y相互独立

D．X与Y同方差

答案

参考答案：C

解析：
若随机变量X1与X2独立，则有Var(X1±X2)=Var(X1)+Var(X2)，所以当X与Y相互独立时，等式Var(X+Y)=Var(X)+Var(Y)成立。

单项选择题

When I was young, my parents ran a snack bar in our small town.One evening in early April, my mother told me to fill in at the snack bar (1) a worker who had the flu. I told her I would mess it up, (2) I had never worked at the bar before. I (3) that instead of making money, I would end up owing it."You can do it," said my mother," (4) , you won’t get much business until lunch. ""But I’ll never remember the orders, and I’m no good (5) money. Please, Mom, don’t (6) me."Then I’ll help you", she said.I shrugged my shoulders. I thought my mother’s (7) was a bad one, but I (8) .When I got to the bar the next day, I found my mother was (9) . Because the weather that day was rainy and cold, people wanted hot snacks and drinks. (10) , I was really slow at taking the orders and making change. The line of people grew, and everybody seemed (11) , I was so nervous that my hands shook, and I (12) a cup into pieces. What a mess! Then my mother came to (13) me, and she also showed me how to make (14) . If someone gave me $ 5 for something that cost $ 3.25, I handed over (15) quarters and a dollar and said, "75 cents makes four dollars, plus one dollar makes five. " Things went more (16) after that.By the end of the day, I could remember orders, (17) the bill, and make change quickly with a smile. I was even a little (18) when the sun came out and dried up business. My mother said she was proud of me, and when she (19) that I work at the snack bar again next year, I did not even shrug. I was too busy (20) the restaurant I would open one day.

 

第（10）空应选择（）

A．At least

B．At last

C．At most

D．At first
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单项选择题

砂浆弹性模量的标准试件高度为（），底模采用钢底模

A.160～180㎜

B.180～200㎜

C.200～220㎜

D.210～230㎜
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