Figure shows a metal rod PQ resting on the smooth rail AB and positioned between the poles of a permanent magnet. The rail, the rod and the magnetic field are in three mutual perpendicular directions. A galvanometer G connects the rails through a switch K. Length of the rod = 15 cm, B = 0.50 T, resistance of the closed loop containing the rod = $9.0 m \Omega$. Assume the field to be uniform. what is the retarding force on the rod when k is closed ?

Question

$(A) \; 7.5 \times 10^{-2} N$
$(B)\; 7.5 \times 10^2 N$
$(C)\; 75 \times 10^2 N$
$(D)\; 75 \times 10^{-2} N$