When the position coordinate x of bc side is in L-2L, the effective tangent length of bC side of wireframe is l=x-L, the induced electromotive force is E=Blv=B(x-L)v, and the induced current is I? =BLvR=B(x？ L)vR, the current gradually increases with x; According to Lenz's law, the direction of induced current is positive, along A → B → C → D → A 。

X is in the process of 2L-3L. According to Lenz's law, it is judged that the direction of induced current is negative edge a→d→c→b→a, the effective tangent length of ad edge of wireframe is l=x-2L, the induced electromotive force is E=Blv=B(x-2L)v, and the induced current I =? BLvR=？ B(x？ 2L)vR, the size of which increases gradually with X, is correct according to mathematical knowledge.

So choose: C.