In the xy-plane, the region $y >0$ has a uniform magnetic field $B_1 \hat{k}$ and the region $y<0$ has another uniform magnetic field $B_2 \hat{k}$. A positively charged particle is projected from the origin along the positive $y$-axis with speed $v _0=\pi m s ^{-1}$ at $t =0$, as shown in the figure. Neglect gravity in this problem. Let $t = T$ be the time when the particle crosses the $x$-axis from below for the first time. If $B_2=4 B_1$, the average speed of the particle, in $m s ^{-1}$, along the $x$-axis in the time interval $T$ is. . . . . . 
  • A$2$
  • B$5$
  • C$10$
  • D$15$
IIT 2018, Advanced
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