A small steel ball of radius $r$ is allowed to fall under gravity through a column of a viscous liquid of coefficient of viscosity $\eta $. After some time the velocity of the ball attains a constant value known as terminal velocity ${v_T}$. The terminal velocity depends on $(i)$ the mass of the ball $m$, $(ii)$ $\eta $, $(iii)$ $r$ and $(iv)$ acceleration due to gravity $g$. Which of the following relations is dimensionally correct

  • A

    ${v_T} \propto \frac{{mg}}{{\eta r}}$

  • B

    ${v_T} \propto \frac{{\eta r}}{{mg}}$

  • C

    ${v_T} \propto \eta rmg$

  • D

    ${v_T} \propto \frac{{mgr}}{\eta }$

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