Which of the following mathematical expressions correctly describes the relationship between the magnitude of the magnetic field (B) outside a straight infinite current-carrying wire of radius 'a', and the distance (r) from the centre of the wire (where r > a)?

A charged particle is released from rest in a region of space where steady and uniform electric and magnetic fields are parallel to each other. The particle will move in a:

Two very long, straight, parallel wires carry steady currents I and 2I in the same direction. The distance between the wires is d. At a certain instant, a point charge q is at a point equidistant from the two wires in the plane of the wires. Its instantaneous velocity v is perpendicular to this plane. The magnitude of the magnetic force acting on the charge at this instant is: