$B_{x}$ and $B_{y}$ are the magnetic fields at the center 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 $B_{x}$ and $B_{y}$ is $:-$

• A
  $1 : 1$
• B
  $1 : 2$
• C
  $2 : 1$
• D
  $4 : 1$