[" (a) "3" " "3" (b) "1/2" .Whe has two concentric loops "],[" An otherwise infinite,straight wire has two concentric loops "],[" of radii a and b carrying equal currents in opposite "],[" directions as shown in Fig.The magnetic field at the "],[" common center is zero for "],[[" (a) "(a)/(b)=(pi-1)/(pi)," (b) "(a)/(b)=(pi)/(pi+1)],[" (c) "(a)/(b)=(pi-1)/(pi+1)," (d) "(a)/(b)=(pi+1)/(pi-1)]]