题目

Mrs. Obama spoke to the graduates of Martin Luther King Jr. High School on Saturday in her only high school commencement（毕业典礼）speech this year. The ceremony took place in the gymnasium of Tennessee State University. The first lady told the 170 graduates that she spent too much of her time in college focusing on academe achievements. While her success in college and law school led to a good job, she said, she ended up focusing on public service. “My message to you today is that don’t waste a minute living someone else’s dream,” she said. “It takes a lot of real wor设a，b，c，是两两不共线的平面向量，则下列结论中错误的是（　　） A、a+b=b+aB、a•b=b•aC、a+（b+c）=（a+b）+cD、a（b•c）=（a•b）c