Bx and By are the magnetic field at the centre of two coils of two coils X and Y respectively, each carrying equal current. If coil X has 200 turns and 20 cm radius and coil Y has 400 turns and 20 cm radius, the ratio of Bx and By is:
(A) 1 : 1
(C) 2 : 1
(B) 1 : 2
(D) 4 : 1