如图是荷质比相同的a、b两粒子从O点垂直匀强磁场进入正方形区域的运动轨迹，则（ ）
A．a的质量比b的质量大
B．a带正电荷、b带负电荷
C．a在磁场中的运动速率比b的大
D．a在磁场中的运动时间比b的长
A、荷质比相同的a、b两粒子，因电量无法确定，则质量大小无法比较．故A错误．
B、初始时刻两粒子所受的洛伦兹力方向都是竖直向下，根据左手定则知，两粒子都带负电．故B错误．
C、根据轨迹图可知，a粒子的半径大于b粒子，根据qvB=m
得，r=v2 r
，则mv qB
=q m
，因它们荷质比相同，即半径越大时，速率越大．故C正确．v Br
D、粒子在磁场中的运动周期T=
，由图可知，a粒子的圆心角大于b粒子，则知a在磁场中的运动时间比b的短．故D错误．2πm qB
故选：C．
| 完形填空。 | |||
| What do you hear? A Native American and his friend were in downtown New York City, walking near Times Square in Manhattan. It was during the lunch hour and the __1__ were filled with people. Cars were honking, taxis were __2__ around corners, sirens (汽笛)were howling, and the sounds of the city were almost __3__. Suddenly, the Native American said,"I hear a cricket." His friend said,"What? You must be __4__. You couldn't possibly hear a cricket in all of this __5__!" "No, I'm sure of it," the Native American said,"I heard a __6__." "That's crazy," said the friend. The Native American listened __7__ for a moment, and then walked across the street to a big cement plant where some bushes were __8__. He looked into the bushes, beneath the branches, and sure enough, he __9__ a small cricket. His friend was utterly __10__. "That's incredible(不可思议)," said his friend."You must have superhuman ears." "No," said the Native American."My __11__ are not different from yours. It all depends on what you're listening for." "But that can't be," said the friend."I could never __12__ a cricket in this noise." "You, it's __13__,"came the reply."It depends on what is really __14__ to you.Here, let me show you."He __15__ into his pocket, pulled out a few coins, and discreetly __16__ them on the sidewalk. And then, even with the noise of the __17__ street still blaring in their ears, they noticed every head within twenty feet __18__ and look to see if the money that tinkled on the __19__ was theirs. "See what I __20__?" asked the Native American."It all depends on what's important to you." | |||
| ( )1. A.supermarkets ( )2. A.waiting ( )3. A.deafening ( )4. A.enthusiastic ( )5. A.environment ( )6. A.siren ( )7. A.carefully ( )8. A.waving ( )9. A.watched ( )10．A.interested ( )11．A.ears ( )12．A.raise ( )13．A.wrong ( )14．A.impressive ( )15．A.reached ( )16．A.distributed ( )17．A.deserted ( )18．A.nod ( )19．A.pavement ( )20．A.perform | B．streets B．parking B．frightening B．cautious B．noise B．horn B．patiently B．planting B．located B．glad B．hands B．feed B．true B．extraordinary B．got B．hid B．crowded B．bend B．counter B．possess | C．restaurants C．screaming C．sickening C．uncertain C．scene C．cricket C．silently C．fading C．followed C．amazed C．eyes C．catch C．foolish C．important C．broke C．placed C．narrow C．turn C．floor C．indicate | D．lanes D．lining D．weak D．crazy D．situation D．call D．nervously D．growing D．played D．lucky D．legs D．hear D．terrible D．necessary D．cut D．dropped D．empty D．lift D．square D．mean |