A point charge is moving with speed $2{10}^7\frac{m}{s}$ parallel to the $x$ axis along strait line at $y=2m.$ At $t=0,$ the charge is at $x=0m.$ The magnitude of the magnetic field at $x=4m$ is $B_0$ at the origin. The magnitude of the magnetic field at at origin when $t=0\mathrm{.}15s$ is

$\frac{B_0}{2}$

$B_0$

$5\frac{B_0}{4}$

$4B_0$

$2B_0$