The loop is divided into upper and lower semicircles, and the magnetic field is weakened. In the upper semicircle and the lower semicircle, a vortex electric field will be generated. According to Lenz's law, the direction of eddy electric field in both circuits is clockwise, so the sum of electromotive force on the diameter is zero, just like the induced current generated by a circular circuit. So the first picture b (that is, the second picture c) is right, and the rest are wrong.