| 如图所示，处在磁感应强度为B的匀强磁场中的单匝矩线圈abcd，以恒定的角速度ω绕ab边转动，磁场方向垂直于纸面向里，线圈所围面积为S，线圈导线的总电阻为R．t=0时刻线圈平面与纸面重合，且cd边正在离开纸面向外运动．则正确的是（ ） A．时刻t线圈中电流的瞬时值i B．线圈中电流的有效值I C．线圈中电流的有效值I= D．线圈消耗的电功率P  |
答案：C
There are different outdoor activities for different seasons. In spring we can fly kites. In summer, we can go swimming or sailing. In autumn, it is a good idea to camp or hike. In winter, we can go to ski or skate because they bring us a lot of fun. They make us strong and fit, too. So it is important for people of all ages to take part in some outdoor activities at the right time. There are many group activities, like football and basketball. The members work together for the same goal（目标）. So group activities need team spirit(团队精神). It is very useful for a person to have team spirit. It is good for us in life and work. Old people like walking and jogging. These outdoor activities don’t need much strength（气力）. Young people like exciting activities like diving（潜水）, skiing and horse riding. Children like fun activities like flying kites. Some outdoor activities are dangerous, like climbing rocks. So we must be careful when we take part in them.
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cosωt
