Two silica particles, one $10$ micron and the other $50$ micron diameter, detach from a ceiling panel at the same time and fall toward the floor in still air.

 $($a$)$ What are the terminal velocities for these particles? 

$($b$)$ Which one reaches its terminal velocity first? Show your reasoning! $($hint:$$you should NOT use ut$/$g to calculate the time to reach terminal velocity as the acceleration is not constant but is actually decreasing due to the increasing drag force$).$