Eight spherical rain drops of the same mass and radius are falling down with a terminal speed of $6\;cm-s^{-1}$. If they coalesce to form one big drop, what will be the terminal speed of bigger drop? (Neglect the buoyancy of the air )

Question

$$\begin {array} {1 1} (1)\;1.5\;cm-s^{-1} & \quad (2)\;6\;cm-s^{-1} \\ (3)\;24\;cm-s^{-1} & \quad (4)\;32\;cm-s^{-1} \end {array}$$