题目

如图所示，水平虚线上有两个等量异种点电荷A、B， M、N、O是AB的垂线上两点，且AO＞OB，2ON＝OM.一个带正电的试探电荷在空间中运动的轨迹如图中实线所示，设M、N两点的场强大小分别EM、EN，电势分别为φM、φN，则下列判断正确的是（     ） A．A点电荷一定带正电 B．试探电荷在M处的电势能小于N处的电势能 C．EM一定小于EN，φM可能大于φN D．UMN＝UNO                     In Kansas City, Missouri, a computer helps fire fighters. The computer, which works very fast, has information about all the 350,000 street addresses in the city. Within two or three seconds after a call is received, the computer provides necessary information for the fire fighters. The information is then sent to them by radio from the computer centre in the City Hall. The exact place of the burning building and its size, type and any other details are clear to them.   The Kansas City computer system also has a medical record of each of the city's 900 fire fighters. This kind of information is especially useful when a fire fighter is injured. With the information, doctors at hospital can treat the injured fire fighters more quickly and easily.   The fire fighters themselves greatly appreciate (感激) the computer's help. They know about possible danger ahead of them and can prepare for it. Many times the computer information helps to save lives and property (财产). Sometimes the lives are those of fire fighters themselves.Mike Horan, a fire fighter captain, says, "I feel as if the computer is watching over me to help me if I get hurt." 41. Which of the following is the best title of the text?   A. Large Computers in Kansas, Missouri.   B. Computers Help Fire Fighters.   C. Important Information for Fire Fighters.   D. The Speed of the Computers. 42. What is the best help if you want to know the information about the street address in Kansas?   A. The fire fighter.                      B. The policeman.   C. The computer.                       D. The map. 43. The fire fighters can get information about a burning place which is sent by   A. the radio                             B. the computer   C. the City Hall                         D. the centre of the city 44. Why can the doctors treat the fire fighters quickly and easily?   A. The hospital has a very good computer.   B. The fire fighters can know about possible danger early.   C. The computers can prepare everything for them.   D. Medical records in the computers help them a lot. 45. From Mike Horan's words, we can learn that the fire fighters themselves   A. are being taken care of by the computers   B. have been saved by the computers many times   C. always get hurt in saving people's lives   D. are trying hard to stop a fire burning