带电粒子（不计重力），以速度v射入某一空间，下列说法正确的是（ ）
A．如果空间只存在匀强电场，则带电粒子穿过该空间时，动能、动量一定发生变化
B．如果空间只存在匀强磁场，则带电粒子穿过该空间时，动能、动量一定发生变化
C．如果空间只存在匀强电场，则带电粒子穿过该空间时，动能可能不变、动量一定发生变化
D．如果空间同时存在匀强电场和匀强磁场，则带电粒子穿过该空间时，动能和动量一定发生变化
A、如果空间只存在匀强电场，则带电粒子穿过该空间时，仅受电场力作用，做匀变速运动，若电场力不做功，动能不变，但是速度的方向一定会发生变化，所以动量一定变化．故A错误，C正确．
B、如果空间只存在匀强磁场，则带电粒子穿过该空间时，因为洛伦兹力不做功，则粒子的动能一定不变．故B错误．
D、如果空间同时存在匀强电场和匀强磁场，则带电粒子穿过该空间时，可能粒子所受的电场力与洛伦兹力平衡，做匀速直线运动，动能和动量都不变．故D错误．
故选C．
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| Jenna, a popular girl from Westwood Middle School, had graduated first in her class and was ready for new 1 in high school. 2 , high school was different. In the first week, Jenna went to tryouts (选拔赛) for cheerleaders (拉 拉队队员). She was competing against very talented girls, and she knew it would be 3 for her to be selected. Two hours later, the 4 read a list of the girls for a second tryout. Her heart 5 as the list ended without her name. Feeling 6 , she walked home carrying her schoolbag full of homework. 7 home, she started with math. She had always been a good math student, but now she was 8 . She moved on to English and history, and was 9 to find that she didn't have any trouble with those subjects. Feeling better, she decided not to 10 math for the time being. The next day Jenna went to see Mrs. Biden about being on the school 11 . Mrs. Biden wasn't as enthusiastic (热情) as Jenna. "I'm sorry, but we have enough 12 for the newspaper already. Come back next year and we'll talk then." Jenna smiled 13 and left. "Why is high school so 14 ?" she sighed (叹气). Later in 15 class, Jenna devoted herself to figuring out the problems that had given her so much 16 . By the end of class, she understood how to get them right. As she gathered her books, Jenna decided she'd continue to try to 17 at her new school. She wasn't sure if she'd succeed, but she knew she had to 18 . High school was just as her mom had said, "You will feel like a small fish in a big pond 19 a big fish in a small pond. The challenge is to become the 20 fish you can be." | ||||
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