An electric current passes through a long straight wire. At a distance 5 cm from the wire, The magnetic field is B. The field at 20 cm from the wire would be [CPMT 2001; Pb PET 2002]

• A.            \[\frac{B}{6}\]          
• B.            \[\frac{B}{4}\]
• C.            \[\frac{B}{3}\]          
• D.            \[\frac{B}{2}\]