下列说法中，不正确的是（ ）
A．质点是一个理想化的模型，实际并不存在
B．因为质点没有大小，所以与几何中的点没有区别
C．被选作参考系的物体是假定不动的
D．质点运动的轨迹是直线还是曲线，与参考系的选取有关
A、质点是一个理想化的模型，实际上并不存在，是感知和思维的产物，故A正确；
B、几何中的点是具体存在点，而质点是理想化的模型实际上是不存在的，故B错误；
C、参考系是指研究物体运动时所选定的参照物体或彼此不作相对运动的物体系，故C正确；
D、参考系的不同会影响物体的相对运动，但是不会影响物体的运动轨迹，故D正确．
本题选错误的，故选B．
Passage Four
| Before the summer of 2000, the 54 year old John Haughom could accomplish just about any thing at work. "I could move mountains if I put my mind to it."he says of those days. But that summer Haughom found he couldn’t move them any more. On the phone with his wife one morn ing, Haughom broke down. A couple of days later Haughom checked himself in for a three-week stay at the Professional Renewal Center, an in-patient clinic 30 miles outside Kansas City that helps him deal with stress. Haughom is far from alone. A host of new studies and plenty of anecdotal evidence show that stress in the workplace is skyrocketing. Whatever the cause, stress levels are at record highs. The statistics are startling. According to a new study by the federal government’s Nation al Institute for Occupational Safety and Health, more than half the working people in the U.S. view job stress as a major problem in their lives. This year the European Community officially dubbed stress the second-biggest occupational-health problem facing the continent. Ten years ago experts warned that stress was out of control, in part because of a shaky economy. What’s notable about today’s wave of stressed-out workers is that it rises all the way to the top. Lack of control is generally considered one of the biggest job stressors, so it used to be thought that middle managers carried the brunt: sandwiched between the top and the bottom, they end up with little authority. Powerful chief executive officers (CEOs) were seen as the least threatened by stress. But in today’s tough economy, top executives don’t have as much control as they used to. "Stress is just part of the job, fortunately or unfortunately, stress’is part of our character building," Lebenthal says. "But I think I don’t need any more character building. What I need is a vacation." But if you think that going on vacation is hard—and studies show that 85%of corporate executives don’t use all the time off they’re entitled to. Being able to handle stress is perhaps the most basic of job expectations. So among the corporate elite, succumbing to it is considered a shameful weakness. Stress has become the last affliction that people won’t dare admit to. Most senior executives who are undergoing treatment for stress—and even many who aren’t—refused to talk on the record about the topic．"Nothing good can come out of having your name in a story like this," one CEO said through his therapist. |
What is this passage mainly about
A．Increasingly serious lack of work places.
B．The second biggest disease in the world.
C．The most serious problem people have to face.
D．Increasingly serious stress faced by working people.