问题
选择题
下列物质中,与乙烯所含碳、氢元素的百分含量相同,但与乙烯既不是同系物又不是同分异构体的是( )
A．环丙烷
B．乙烷
C．甲烷
D．CH3CH=CH2
答案
答案：A
题目分析：乙烯分子式是C2H4，若物质与乙烯所含碳、氢元素的百分含量相同,则应该是具有相同的分子式通式；排除BC选项；但与乙烯既不是同系物又不是同分异构体，则又排除D选项。故答案是A。
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下列物质中,与乙烯所含碳、氢元素的百分含量相同,但与乙烯既不是同系物又不是同分异构体的是( )
A．环丙烷
B．乙烷
C．甲烷
D．CH3CH=CH2
答案：A
题目分析：乙烯分子式是C2H4，若物质与乙烯所含碳、氢元素的百分含量相同,则应该是具有相同的分子式通式；排除BC选项；但与乙烯既不是同系物又不是同分异构体，则又排除D选项。故答案是A。
| A pilot cannot fly a plane by sight alone. In many conditions, such as flying at night and landing in dense fog, a pilot must use radar, an alternative way of navigating. Since human eyes are not very good at determining speeds of approaching objects, radar can show a pilot how fast nearby planes are moving. The basic principle of radar is exemplified by what happens when one shouts in a cave. The echo of the sounds against the walls helps a person determine the size of the cave. With radar, however, the waves are radio waves instead of sound waves. Radio waves travel at the speed of light, about 300, 000 kilometers in one second. A radar set sends out a short burst of radio waves: Then it receives the echoes produced when the waves bounce off objects. By determining the time it takes for the echoes to return to the radar set, a trained technician can determine the distance between the radar set and other objects. The word "radar," in fact, gets its name from the term "radio detection and ranging." "Ranging" is the term for detection of the distance between an object and the radar set. Besides being of critical importance to pilots, radar is essential for air traffic control, tracking ships at sea, and for tracking weather systems and storms. |
What is the main topic of this passage
A．The nature of radar.
B．Types of ranging.
C．Alternatives to radars.
D．History of radar.