Two long parallel straight conductors carry equal currents I in opposite directions, separated by a distance d. Four closed paths are chosen in the plane perpendicular to the conductors:
(a) a circle of radius 4d around conductor A only,
(b) a circle of radius 4d around conductor B only,
(c) a circle of radius 2d centred midway between the conductors, and
(d) a continuous figure-eight loop that goes once around A and once around B in opposite senses.
For which path(s) will the value of the line integral ∮B⋅dl be zero?
a) Paths (c) and (d)
b) Path (d) only
c) Path (c) only
d) Paths (a) and (b) only