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写出以下单词的中文意思：command n.

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从供选择的答案中选出应填入下面一段英文中______内的正确答案。
A light-emitting diode (LED) is a semiconductor(1)that emits visible light when an electric current passes throughit.The light is not particularly(2), but in most LEDs it is monochromatic, occurring at a single(3). The output from an LED can range from red (at a wavelength of approximately 700 nanometers) to blue-violet (about 400 nanometers). Some LEDs(4)infrared (IR) energy (830 nanometers or longer); such a device is known as an infrared-emitting diode (IRED).
An LED or IRED(5)of two elements of processed material called P-type semiconductors and N-type semiconductors. These two elements are(6)in direct contact, forming a region called the P-N junction. In this respect, the LED or IRED(7)most other diode types, but there are important(8). The LED or IRED has a transparent package, allowing(9)or IR energy to pass through. Also, the LED or IRED has a large PN-junction area whose(10)is tailored to the application.
供选择的答案：
1．wavelength 2．shape 3．emit 4．device 5．resembles
6．bright 7．visible 8．consists 9．differences 10．placed

写出以下单词的中文意思：pulse n.

根据给出的短语，写出中文意思：result in

根据给出的中文意思，写出短语：中心交换