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问题 单项选择题

实时操作系统必须首先考虑的是( )。

A．高效率

B．及时响应和高可靠性、安全性

C．多路性和交互性

D．“多道”和“成批”

答案

参考答案：B

解析： 在实时操作系统中，实时的含义是对随机发生的外部事件做出及时响应并对其进行处理。实时系统的特点是对外部信号必须及时响应，即每一个信息接收、分析处理和发送的过程必须在严格的时间限制内完成；其另一个主要特点是要有高可靠性，系统发出的信号或响应的信息必须是绝对可靠的和安全的，而系统的效率则是第二位的。
“多道”和“成批”足批处理操作系统的特点；多路性和交互性是分时系统的特点。

单项选择题

第三篇How the First Stars in the Universe Came into ExistenceResearchers believe that our universe began with the Big Bang about 13 billion years ago, and that soon after that event, matter began to form as small dust grains and gases. How the first stars formed from this dust and gas has been a burning question for years, but a state-of-the-art computer simulation now offers the most detailed picture yet of how these first stars in the universe came into existence.The composition of the early universe was quite different from that of today, and the physics that governed the early universe were also somewhat simpler. Dr. Naoki Yoshida and colleagues in Japan and the U.S. incorporated these conditions of the early universe, sometimes referred to as the “cosmic dark ages,” to simulate the formation of an astronomical object that would eventually shine its light into this darkness.The result is a detailed description of the formation of a protostar — the early stage of a massive primordial star of our universe — and the researchers’ computer simulation sets the bar for further investigation into the star formation process. The question of how the first stars evolved is so important because their formations and eventual explosions provided the seeds for subsequent stars to come into being.According to their simulation, gravity acted on minute density variations in matter, gases, and the mysterious “dark matter’’ of the universe after the Big Bang in order to form this early stage of a star — a protostar with a mass of just one percent of our sun. The simulation reveals how pre-stellar gases would have actually evolved under the simpler physics of the early universe to form this protostar. Dr. Yoshida’s simulation also shows that the protostar would likely evolve into a massive star capable of synthesizing heavy elements, not just in later generations of stars, but soon after the Big Bang.“This general picture of star formation, and the ability to compare how stellar objects form in different time periods and regions of the universe, will eventually allow investigation into the origins of life and planets,” said Lars Hernquist, a Professor of Astronomy at Harvard University. “The abundance of elements in the universe has increased as stars have accumulated,” he says, “and the formation and destruction of stars continues to spread these elements further across the universe. So when you think about it, all of the elements in our bodies originally formed from nuclear reactions in the centers of stars, long ago.’’Their simulation of the birth of a protostar in the early universe signifies a key step toward the ambitious goal of piecing together the formation of an entire primordial star and of predicting the mass and properties of these first stars of the universe. More powerful computers, more physical data, and an even larger range will be needed for further calculations and simulations, but these researchers hope to eventually extend this simulation to the point of nuclear reaction initiation — when a stellar object becomes a true star.

What does the "astronomical object" in paragraph 2 refer to

A．cosmic dark ages.

B．dust grains and gases.

C． a protostar

D．the early universe.
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单项选择题

由于改制、搬迁等特殊情况需要延长许可证有效期时，许可证延长期一般不超过（）.

A、半年

B、一年

C、两年
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