A Very Long Straight Current-Carrying Wire Produces a Magnetic Field $20 \mathrm { mT }$

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

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