In a co-axial, straight cable, the central conductor and the outer conductor carry equal currents in opposite directions. The magnetic field is zero

The magnetic field, of a given length of wire for single turn coil, at its centre is B, then its value for two turns coil for the same wire is

In which of the following case no force exerted by a magnetic field on a charge?

A long thin hollow metallic cylinder of radius R has a current i ampere. The magnetic induction B away from the axis at a distance r from the axis varies as shown in