题目内容
在无线通信中不能采用CSMA/CD方式，这是因为无线通信存在(59)，采用CSMA/CA技术是指(60)，MAC层的分布式协调功能提供(61)，站点发出的确认帧ACK，要在(62)间隔发送。为解决隐蔽站带来的问题，802.11允许(63)。
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听力原文： Nowadays, at each check-out counter in almost all supermarkets, a new machine is helping customers in a hurry. An electronic system "reads" a bar code on each tin, box or bottle. These little lines of the bar code give the computer all the information it needs -- the kind of product, the price and the tax. The girl at the counter simply moves the groceries along in front of the "eye" of the machine. And the bill is ready in no time.
Most people are very pleased with this new idea. They don't make as many mistakes as people. Some shoppers, how ever, are complaining. They can’t read the price of the food on a tin when all they can see is just little black and white lines. The prices are on the shelves, but they want them on each tin. Many people are not sure that machines are honest shopkeepers! The machines are very expensive. But everyone, even these worried shoppers, will be happy, if in the end, they bring food prices down.
The main idea of this passage is that ______.
在航空运输中，若一票货物包括多种物品，托运人填写品名时可以使用“样品”“部件”等名称。
A．将硬件相关和无关的部分分开B．便于实现C．便于网络管理D．因为下层用硬件实现
A．他个人B．所在单位C．单位与个人都是D．无法判断
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