Four condensers each of capacitance $8 \mu \mathrm{~F}$ are joined as shown in the figure. The equivalent capacitance between the points $A$ and $B$ will be

A steady current is flowing in a metallic conductor of non-uniform cross-section. The physical quantity which remains constant is

The resistance between points $A$ and $C$ in the given network is

A wire shaped in a regular hexagon of side 2 cm carries a current of 4 A . The magnetic field at the centre of hexagon is