题干

粒子回旋加速器的工作原理如图所示,置于真空中的D型金属盒的半径为R,两金属盒间的狭缝很小,磁感应强度为B的匀强磁场与金属盒盒面垂直,高频率交流电的频率为f,加速电压的电压为U,若中心粒子源处产生的质子质量为m,电荷量为+e,在加速器中被加速.不考虑相对论效应,则下列说法正确是(   )

A:质子第二次和第一次经过D型盒间狭缝后轨道半径之比为 2 :1

B:加速的粒子获得的最大动能随加速电场U增大而增大

C:质子被加速后的最大速度不能超过2πRf

D:不改变磁感应强度B和交流电的频率f,该加速器也可加速α粒子

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A,C

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    For the first time in the history of the Beijing, Shanghai and Hangzhou marathons, fourteen blind runners competed in the Nov 6 Hangzhou Marathon this year with the assistance of personal guides. Each runner was supported by 4 or 5 volunteers. Among them was Yan Wei, from a running group called “Running in the Dark”, which specially provides training for blind runners.

    During trainings, blind runners are each accompanied by three volunteers. One volunteer is responsible for removing any obstacles(障碍物)on the road and controlling the pace. The second volunteer acts as a lookout for other runners or vehicles on the path while the third, who is attached to the blind runner by a safety rope, takes note of the trainee's running gait and pace.

    Zhu Peihua, the leader of the group's Shanghai branch, first developed a passion for running last year when he took part in a guided running activity. Zhu said that he was so fascinated with the sport that he even bought a treadmill(跑步机)so that he could train at home. “Running gives me the chance to be outside and take in the fresh air. The activity has also made me more willing to talk to people. It's simply good for health, ” said Zhu.

    The popularity of guided running activities for blind people has been growing since last year. Cheng Yi, a volunteer at Running in the Dark's Hangzhou branch, said that both the numbers of blind runners and volunteers to become assistants to them are rising continually. However, he added that volunteers face a more physically demanding task compared to a regular runner. Some of the other challenges involve being aware of the verbal(口头的) instructions one gives.

     “At the beginning, I kept saying ‘here' and ‘there' instead of ‘left' or 'right' which is much more specific and relevant to the runner. A blind runner once collided with another runner because of my mistake. Luckily, no one was hurt,” said Cheng. Through everything, according to Cheng, he and many of the blind runners he has guided become close friends with shared topics and increased understandings.