一质点做圆周运动，速度处处不为零，则（） A．任何时刻质_爱下问搜题

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问题多选题

一质点做圆周运动，速度处处不为零，则（　　）

A．任何时刻质点所受的合力一定为零

B．任何时刻质点的加速度一定不为零

C．质点速度的大小一定不断地变化

D．质点速度地方向一定不断地变化

答案

A、做圆周运动的物体速度方向一定变化，一定具有加速度，则合外力一定不为零．故A错误，B正确．

C、圆周运动的质点速度大小可能改变，可能不变．故C错误．

D、某点的切线方向即为质点速度方向，所以圆周运动速度方向不断变化，故D正确；

故选BD．

完形填空

When I was little, my mother began selling flowers in Camden Market. She couldn’t 36 me at home on my own,   37 I had to go to the market with her.
38 , I didn’t have to do anything. I just sat there, and my mother told me jokes and stories so I didn’t get   39 .When I got older, I gave my mother a hand and I really enjoyed it.
Then one year, my grandma gave me a book for Christmas. It was all about making candles and I loved it. I was 14, and I didn’t have to go to the market 40 because my mother could leave me at home on my own. I 41 my free time making candles of all shapes and sizes. I made 42 them.
One day, my mother was ill, so I had to go to the market on my own. I decided to 43 some candles with me and see 44 I could sell them. They 45 in 20 minutes! The next week, my mother gave me some money to buy some wax (蜡) to make more candles. They were sold out really quickly, too.
Ten months later, we decided 46 flowers. My mother and I couldn’t make enough candles during the week, so some of my friends started to help us. I paid them one pound 47 every candle, and we   48 sell them for three or four times. It was fun and my friends worked with me in the market.
At the age of 22, my uncle 49 me some money and I opened my first shop in Portobello Road. Since then, I’ve 50 looked back, even in difficulties. In the first store, we only sold candles, but now we sell everything from the designer, furniture to paintings. Oh, and candles, of course.

A．To begin with

B．To begin to

C．To start to

A．hundreds of

C．were sold out

A．stop selling

B．to stop selling

C．to stop to selling

D．to stop to sell

A．are used for

C．are used to

问答题

设乙发现甲的电脑比约定的标准差得多后，拒绝购买，甲也不同意降价，双方遂解除合同。则乙往返于C地和D地的费用应如何处理