The magnetic field at the centre of a circular coil of radius r carrying current I is \[{{B}_{1}}\]. The field at the centre of another coil of radius 2 r carrying same current I is \[{{B}_{2}}\]. The ratio \[\frac{{{B}_{1}}}{{{B}_{2}}}\] is [MP PET 2001]

• A.   \[\frac{1}{2}\]    
• B.   1
• C.   2    
• D.   4