设集合A={x|y=x2-1}，B={y|y=x2-1}，C={（x，y）|y=x2-1}，则下列关系中不正确的是（ ）
A．A∩C=∅
B．B∩C=∅
C．B⊆A
D．A∪B=C
由题意可知，集合A=R；集合B中的函数y=x2-1≥-1，所以集合B=（-1，+∞）；
而集合C中的元素为二次函数y=x2-1图象上任意一点的坐标．
则A∩C=∅，B∩C=∅，且B⊆A，所以答案D错误，
故选D
| President Barack Obama shouldn't be surprised if his approval rating among students has plummeted (陡然变差). In a recent speech about US education, he called for extending the school day and year. The challenges of a new century demand more time in the classroom. Why more time? Because US students are falling behind students in other countries, he said. More school US schools need to lengthen school days and the school year. If we intend to stay competitive with the rest of the world on test scores and in the job market, we need to add classroom hours and get rid of summer break almost entirely. As President Obama said, South Korean students spend more than a month longer in school-and achieve better results than us. The current twoterm school year and the school days that form it were developed based on an old system of agriculture. The long summer break allowed children to be home to help tend the fields. It is not necessary anymore. It may not be popular to cut summer break or extend school days, but that's the only way to improve US education and get back the American jobs lost to foreign countries. Home Time Adding school hours won't solve the problems with our school system. The USA is different from other countries. US families have full schedules, with participation in sports, community groups, and other interests. In places such as South Korea, where only several hours of a day aren't spent on school work, children don't have time to participate in many of those activities. Giving kids time outside of school allows them to grow in other areas. That's just as important as learning fractions (分数). Fingers shouldn't be pointed at the school year for being too short, but at teachers and schools that don't challenge their students and just pass kids along to the next grade. We do need to hold ourselves accountable for an education system falling behind other countries, but we also need a solution that actually works.
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