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使用WWW浏览器浏览网页时，用户可用鼠标点击某个超链接，从协议分析的角度看，此时，浏览器首先需要进行（）。
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In everyday usage "hot" means "having a lot of heat". Many people think that "cold" is something completely separated from heat. But this is not true. Cold simply means "having very little heat". Your life depends on heat. In fact, every living thing depends on it. Without heat, every living thing would be frozen to death. All living things get their heat from the sun, which provides the conditions in which life is possible.Since before the dawn of history, man has been able to make his own heat. He has been able to release the sun"s heat that is trapped in things such as wood, coal and oil. And he has been able to use this heat.Heat has made civilization possible. With heat, man could melt metals. As man learned to use metals and fuels, industries grew. As a result, engines were invented. These are machines that change heat energy into mechanical energy. Engines can do the work of many men. Without engines industrial civilization is impossible.Yet when the first engines were built in the 17th century, men were still wondering about the nature of heat. "What is it" they asked. Not until the early years of the 19th century did they find the right answer. In everyday usage, "hot" means ______.
横截面尺寸为25mm×10mm的矩形波导中填充介质为空气，若要实现单模传输，则电磁波的工作频率范围为______。
在电介质中，电场强度E的散度为零之处，也可能存在自由电荷。
发生全反射时，透射系数τ不一定等于零。
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