题目

Many people believe the glare from snow causes snow-blindness. Yet, dark glasses or not , they find themselves suffering from headaches and watering eyes, and even snow-blindness, when exposed to several hours of “snow light”. The United States Army has now determined that glare from snow does not cause snow-blindness in troops in a snow-covered country. Rather, a man's eyes frequently find nothing to focus on in a broad expanse of barren (少植被的) snow-covered terrain (地形). So his gaze continually shifts and jumps back and forth over the entire landscape in search of something to如图所示，在磁感应强度为B的匀强磁场中有固定的金属框架ABC，已知∠B=θ，导体棒DE在框架上从B点开始在外力作用下，沿垂直DE方向以速度v匀速向右平移，使导体棒和框架构成等腰三角形回路。设框架和导体棒材料相同，其单位长度的电阻均为R0，框架和导体棒均足够长，不计摩擦及接触电阻，关于回路中的电流I和电功率P随时间t变化的下列四个图像中可能正确的是（ ）