单项选择题 Bluetooth When you use computers, entertainment systems or telephones, the various pieces and parts of the systems make up a community of electronic devices. These devices communicate with each other using a variety of wires, cables, radio signals and infrared light (红外线) beams, and an even greater variety of connectors and plugs. The art of connecting things is becoming more and more complex every day. And Bluetooth has now become a new popular method of connecting devices that can streamline the process. A Bluetooth connection is wireless and automatic, and it takes small-area networking to the next level by removing the need for user intervention and keeping transmission power extremely low to save battery power.Bluetooth Operation Bluetooth networking transmits data via low-power radio waves. It communicates on a frequency of 2.45 gigahertz (千兆赫) (actually between 2.402 GHZ and 2. 480 GHz, to be exact). This frequency band has been set aside by international agreement for the use of industrial, scientific and medical devices (ISM). A number of devices that you may already use take advantage of this same radio-frequency band. Baby monitors (微型监视器), garage-door openers and the newest generation of cordless phones (无绳电话) all make use of frequencies in the ISM band. One of the ways Bluetooth devices avoid interfering with other systems is by sending out very weak signals of about 1 milliwatt. By comparison, the most powerful cell phones can transmit a signal of 3 watts. The low power limits the range of a Bluetooth device to about 10 meters (32 feet), cutting the chances of interference between your computer system and your portable telephone or television. Even with the low power, Bluetooth doesn't require line of sight between communicating devices. The wall in your house won't stop a Bluetooth signal, making the standard useful for controlling several devices in different rooms. Bluetooth can connect up to eight devices simultaneously. With all of those devices in the same 10-meter (32-foot) radius, you might think they'd interfere with one another, but it's unlikely. Bluetooth uses a technique called spread- spectrum (扩展频谱) frequency that makes it rare for more than one device to be transmitting on the same frequency at the same time. In this technique, a device will use 79 individual, randomly chosen frequencies within a designated range, changing from one to another on a regular basis. In the case of Bluetooth, the transmitters change frequencies 1,600 times every second, meaning that more devices can make full use of a limited slice of the radio spectrum. Since every Bluetooth transmitter (传输设备) uses spread-spectrum transmitting automatically, it's unlikely that two transmitters will be on the same frequency at the same time. This same technique minimizes the risk that portable phones or baby monitors will disrupt Bluetooth devices, since any interference on a particular frequency will last only a tiny fraction of a second.Piconets (个人域网络) Let's say you have a typical modern living room with the typical modern stuff inside. There's an entertainment system with a stereo, a DVD player, a satellite TV receiver and a television; there's also a cordless telephone and a personal computer. Each of these systems uses Bluetooth, and each forms its own piconet to talk between the main unit and peripheral (外围设备). The cordless telephone has one Bluetooth transmitter in the base and another in the handset. The manufacturer has programmed each unit with an address that falls into a range of addresses it has established for a particular type of device. When the base is first turned on, it sends radio signals asking for a response from any units with an address in a particular range. Since the handset has an address in the range, it responds, and a tiny network is formed. Now, even if one of theA．YB．NC．NG

单项选择题 提高国家竞争力的核心和关键是（ ）

单项选择题 邓小平指出：“计划和市场都是方法嘛。只要对发展生产力有好处，就可以利用。它为社会主义服务，就是社会主义为资本主义服务，就是资本主义的。”这些论断明确界定计划和市场的属性（ ）