A Very Long Straight Current-Carrying Wire Produces a Magnetic Field $\sqrt { 2 }$

A very long straight current-carrying wire produces a magnetic field of 20 mT at a distance d from the wire. To measure a field of 5 mT due to this wire, you would have to go to a distance from the wire of

A) 16d.
B) 8d.
C) 4d.
D) 2d.
E) d $\sqrt { 2 }$