If the key in the arrangement shown in figure is taken out (the circuit is made open) and magnetic field lines are drawn over the horizontal plane
ABCD, the lines are
(a) concentric circles
(b) elliptical in shape
(c) straight lines parallel to each other
(d) concentric circles near the point O but of elliptical shapes as we go away from it
Explanation : When no current flows through the wire, mag¬netic field lines over the plane ABCD will represent magnetic field of the earth. These field lines are straight lines parallel to each other.