题目

Late research shows that yawning helps in keeping the brain cool, contradicting the popular belief that yawning promotes sleep and is a sign tiredness.      Yawning involves opening the mouth involuntarily while taking a long, deep breath of air. It is commonly believed that people yawn as a result of sleepiness or tiredness because they need oxygen.      However, researchers at the university of Albany in New York said their experiments on 44 students showed that drawing in air helps cool the brain and helps it work more effectively.      They said that their experiments showed th一个物体沿圆形轨道运动，在相等时间内通过的路程相等，则物体的运动状态 [     ]A．不断改变?  B．始终不变  C．有时改变，有时不改变?  D．无法确定