题目

如图所示，等离子气流（由高温高压的等电量的正、负离子组成）由左方连续不断地以速度射入P1和P2两极板间的匀强磁场中，直导线与P1、P2相连接，线圈A与直导线cd相连接。线圈A内存在变化的磁场，且磁感应强度B的正方向规定为向左，已知ab和cd的作用情况为：0～2s内互相排斥，2～4s内互相吸引。则线圈A内磁感应强度B随时间变化的图象不可能是（    ）Attention! Students ! Beach party comes ! We are going to have an outdoor party at the National Park on July 15. Activities: 1. Ride bikes from the Student Center to the beach. 2. A picnic on the beach. 3. Swimming and sailing contests.(帆船赛) What to bring: sun cream, sunglasses, hats, swimsuits, hiking shoes and personal items. Deadline for signing up(报名): Sunday, July 8 Fee: 200 per person. Meet at the Student Center at 10:00 am, July 15 P. S：All the food will be offered by the Students’ Union. 1. Where do you usually find a poster like this?      A. On the beach       B. In a bookstore      C. In a school         D. In a park 2.Students don’t need to bring _______.       A. swimsuits    B. sunglasses    C. food    D. hiking shoes 3. Where do the students meet?       A. At the student Center.                 B. At the National Park       C. On the beach                        D. At the Students Union 4.Which of the following is WRONG?       A. Boats are needed in the activities    B. The party begins at 10:00 pm       C. The last day to signup is July 8     D. Students will have a picnic on the beach 5.What’s the main purpose (目的) of the poster?       A. To tell students what to do in the summer.    B. To teach students how to plan a party.   C. To make money for National Park. D. To get more students to join the activities if possible.