As shown in the figure, the magnetic induction intensity of the uniformly distributed magnetic field B changes uniformly with time, that is, B=Kt(K is a constant), and there are two same species in the magnetic field with the same thickness. According to Faraday's law of electromagnetic induction, the induced electromotive force E= △? △t = △B？ △t S =KS

Then from ohm's law: the magnitude of induced current is I= E r = KS r = K? π r 2 r =Kπr, so b is correct and ACD is wrong;

So choose: B.