3岁女孩，1周前曾患感冒，1d来发现全身散发瘀点和瘀斑，以双下肢多见。病后不发热。体检：肝、脾不大。门诊查血小板数40×109／L，诊断为原发性血小板减少性紫癜，急性型。其发生出血的机制不包括（）
A.VonWillebrand因子缺乏
B.抗体损伤血管壁
C.血小板数量减少
D.血小板聚集力、粘附力减弱
E.血小板第3因子活性减低
参考答案：A
Passage Two
| Cars account for half the oil consumed in the U.S. , about half the urban pollution and one fourth the greenhouse gases. They take a similar toll (损耗) of resources in other industrial nations and in the cities of the developing world．As vehicle use continues to increase in the coming decade, the U.S. and other countries will have to deal with these issues or else face unacceptable economic, health-related and political costs. It is unlikely that oil prices will remain at their cur- rent low level or that other nations will accept a large and growing U. S. contribution to global climatic change. Policymakers and industry have four options: reduce vehicle use, increase the efficiency and reduce the emissions of conventional gasoline-powered vehicles, switch to less harmful fuels, or find less polluting driving systems. The last of these—in particular, the introduction of vehicles powered by electricity-is ultimately the only sustainable (可持续的) option The other alternatives are attractive in theory but in practice are either impractical or offer only marginal improvements. For example, reduced vehicle use could solve traffic problems and a host of social and environmental problems, but evidence from around the world suggests that it is very difficult to make people give up their cars to any significant extent. In the U.S. mass-transit rider ship and carpooling (合伙用车) have declined since World War Ⅱ. Even in western Europe, with fuel prices averaging more than＄1 a liter (about ＄ 4 a gallon) and with easily accessible mass transit and dense populations, cars still account for 80 percent of all passenger travel. Improved energy efficiency is also appealing, but automotive fuel economy has barely made any progress in 10 years. Alternative fuels such as natural gas, burned in internal-combustion engines, could be introduced at relatively low cost, but they would lead to only marginal (边缘的最低限度的) reductions in pollution and greenhouse emissions (especially because oil companies are already spending billions of dollars every year to develop less polluting types of gasoline). |
The U.S. has to deal with the problems arising from vehicle use because______.
A．most Americans are reluctant to switch to public transportation systems
B．the present level of oil prices is considered unacceptable
C．other countries will protest its increasing greenhouse emissions
D．it should take a lead in conserving natural resources
