某同学设计了如右图所示的双向开关电路，电源电压保持不变，灯Ll标有“3V 3W”字样，灯L2标有“9V 4.5W”字样.当开关拨至2位置且滑片滑到A端时，灯L2恰好正常发光．下列说法中正确的是（不计灯丝电阻随温度的变化）（ ）
A．电源电压为12V
B．L2正常发光时，电路中的电流为2A
C．要使L1正常发光，滑动变阻器接入电路的阻值应为6Ω
D．当灯L1、L2分别正常发光时，两次电路的总功率之比为2:3
答案：C
题目分析：当开关拨至2位置且滑片滑到A端时，仅L2接入电路，灯L2恰好正常发光，故电源电压为9V，通过的电流I=P/U=4.5W/9V=0.5A；L1正常发光时，由题意知：R1=U12/P1=（3V）2/3W=3Ω，电流I=P1/U1=3V/3W=1A，电源电压U=9V，则此时电路总电阻R=U/I=9V/1A=9Ω，滑动变阻器接入电路的阻值RP=R－R1=9Ω－3Ω=6Ω；故正确选项是C。
Passage Two
| During the last thirty years, the international economy has experienced a basic change. Improvements in the Internet and other communications have had important effects on world markets. Faster methods of transport from one place to another have made the world seem smaller to the businessmen. As a result, the world no longer. consists of a number of separate economies under the control of different nations. Instead, the nations have been integrated into a single economy, and the integration is becoming more and more complete every year. For the first time in history, we can truly speak of a global economy. The most dramatic example of this integration was the oil crisis of 1973 and 1974. It came as a great surprise to the public in the industrial nations to discover that they depended so heavily on imported oil. However, the best evidence for the growing integration is the rapid expansion in the volume of world trade. It went up by about 7% a year during the decade from 1990 to 2000, and in several quasi-industrial (半工业的) countries the growth was even more rapid. As a result, some imported products have become as common as domestic commodity. Some of them are too common for the public to any more realize they are foreign. Production has also become international, which is manifested by the large corporations stepping across national borders and established branches and subsidiaries in several different countries. As an example, U.S. companies are building automobiles in Canada, Germany, Britain and Japan. In some cases, components of an automobile are produced abroad and shipped to the United States, where it is then finished with the imported parts. When even the United States has the largest number of such corporations, it is not the only. Other multinationals, for instance, are based in Japan, France, Germany, the UK and Italy. Labor, too, is much more mobile than in the past. Both skilled and unskilled workers can now readily migrate from one country to another. In Europe, for example, there are large numbers of Turkish workers employed in the Germany Economy. Doctors, lawyers, and other professionals are also finding it easier to work in foreign countries. The labor market has become international, and the number of expatriate workers is continuing to grow. |
We learn from the passage that ______ .
A．some less developed countries maintain a faster growth rate
B．the United States has more international corporations than other countries
C．the U.S. automobile industry is collapsing under competitive pressure from Japan and other industrialized countries
D．skilled workers and professionals find it easier to work abroad than unskilled workers