When the coil enters the magnetic field, as shown in the figure, the effective cutting length is l = 2a2? (a？ x)2

So when the ring just entered the magnetic field: I = blvr = 2ba2? (a？ X)2vR, when the ring enters the right magnetic field, the two parts cut the magnetic induction line at this time, and the current becomes twice the original. According to the mathematical relationship, ABD is wrong and C is right.

So choose C.