The magnetic field at the centre of a circular current carrying-conductor of radius $r$ is $B_c$. The magnetic field on its axis at a distance $r$ from the centre is $B_a$. The value of $B_c : B_a$ will be :-

  • A

    $1: \sqrt 2$

  • B

    $1:2 \sqrt 2$

  • C

    $2 \sqrt 2 :1 $

  • D

    $\sqrt 2 :1 $

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