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 ${\upsilon _T}$. 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